Chicago (CNN) A year after unrest in Ferguson, Mo., brought increased scrutiny of police, FBI Director James Comey has thrown his weight behind the idea that restraint by cops in the wake of criticism is at least partly to blame for a surge in violent crime in some cities.

The tensions over policing and crime come when, for the first time in a generation, unusual political forces have aligned and the nation appears on the verge of relaxing tough criminal sentencing laws. Liberals and conservatives now seem to agree that 1980's-era anti-drug laws boosted U.S. prison populations too much, with the burden falling disproportionately on minority communities.

Some law enforcement officials, including Comey, are raising concerns that a spike in crime -- or at least the perception that the recent era of historically low crime rates is at risk -- could hurt criminal justice reforms efforts.

People walk past a plaque honoring Brown following the memorial service in Ferguson on August 9.

People walk past a plaque honoring Brown following the memorial service in Ferguson on August 9.

Brown supporters wear T-shirts with the late teen's picture and the message "Chosen 4 Change" on August 9 as they take part in the remembrance ceremony in Ferguson.

Brown supporters wear T-shirts with the late teen's picture and the message "Chosen 4 Change" on August 9 as they take part in the remembrance ceremony in Ferguson.

People attend a memorial service outside Canfield Apartments in Ferguson on August 9. Several hundred demonstrators stood in silence at the spot where Brown was shot and killed.

People attend a memorial service outside Canfield Apartments in Ferguson on August 9. Several hundred demonstrators stood in silence at the spot where Brown was shot and killed.

Police face off with demonstrators in Ferguson on August 9.

Police face off with demonstrators in Ferguson on August 9.

Demonstrators stage a "die-in" along West Florissant Avenue in Ferguson on August 9.

Demonstrators stage a "die-in" along West Florissant Avenue in Ferguson on August 9.

Protesters march in the rain in Ferguson on August 9.

Protesters march in the rain in Ferguson on August 9.

A protester yells in front of a line of police in Ferguson on August 9.

A protester yells in front of a line of police in Ferguson on August 9.

Demonstrators march along West Florissant Avenue in Ferguson on August 9. West Florissant is not far from Canfield Drive, where Brown was shot a year ago.

Demonstrators march along West Florissant Avenue in Ferguson on August 9. West Florissant is not far from Canfield Drive, where Brown was shot a year ago.

Police take cover after gunfire in Ferguson on August 9.

Police take cover after gunfire in Ferguson on August 9.

A man with gunshot wounds lies face down in a parking lot after a shootout with police in Ferguson on Sunday, August 9. Demonstrations started peacefully, but gunfire erupted Sunday night. Police say the man fired at them.

A man with gunshot wounds lies face down in a parking lot after a shootout with police in Ferguson on Sunday, August 9. Demonstrations started peacefully, but gunfire erupted Sunday night. Police say the man fired at them.

Cornel West, center, joins other protesters sitting on the steps of the Thomas F. Eagleton Federal Courthouse as members of the Federal Protective Service stand watch August 10 in St. Louis. Protesters have been arrested after blocking the entrance to the courthouse while calling for more aggressive U.S. government response to what they call racist law enforcement practices.

Cornel West, center, joins other protesters sitting on the steps of the Thomas F. Eagleton Federal Courthouse as members of the Federal Protective Service stand watch August 10 in St. Louis. Protesters have been arrested after blocking the entrance to the courthouse while calling for more aggressive U.S. government response to what they call racist law enforcement practices.

Protesters march to the Federal Courthouse on August 10 in St. Louis.

Protesters march to the Federal Courthouse on August 10 in St. Louis.

A demonstrator is arrested in Ferguson on August 10.

A demonstrator is arrested in Ferguson on August 10.

Members of a group calling themselves the "Oath Keepers" appear in Ferguson carrying large guns on Monday, August 10, one day after a police confrontation led to a protester being shot. Sunday, August 9, marked the one-year anniversary of the death of Michael Brown.

Members of a group calling themselves the "Oath Keepers" appear in Ferguson carrying large guns on Monday, August 10, one day after a police confrontation led to a protester being shot. Sunday, August 9, marked the one-year anniversary of the death of Michael Brown.

The crime spike is showing up in a variety of cities big and small, while others have avoided the same. Cleveland and Milwaukee have blown past the number of murders reported in 2014, with more than two months left in the year. Dallas and Tampa in recent weeks were on pace to surpass 2014 murder totals. Meanwhile, other cities, including New York, haven't seen similar increases.

At the same time, a number of high-profile police shooting incidents, many caught on ubiquitous camera phones, have given rise to protests over policing tactics that critics call heavy-handed. In some cities, police officers privately report holding back on making stops for fear of ending up the next YouTube "bad cop" sensation. They call it the Ferguson effect.

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These are among the issues facing the nation's top local and federal law enforcement officials meeting in Chicago this week. President Barack Obama and Attorney General Loretta Lynch are scheduled to address the gathering of the International Association of Chiefs of Police.

As if to underscore the roiling controversies hanging over policing in America, on Saturday hundreds of protesters staged what they called an "I Shocked the Sheriff" Counter-Conference outside the Chicago convention hall hosting the IACP meetings. Police made dozens of arrests when protesters blocked intersections.

Comey waded into the thorny issues at play in a speech Friday at the University of Chicago Law School, his alma mater. He has expressed worry about the spike in the number murders in some cities, and for the first time said it could be at least partly linked to what he called a "chill wind" police are facing in the wake of Ferguson.

Photos: Photos: Emotions run high in Ferguson Photos: Photos: Emotions run high in Ferguson Protesters march in Ferguson, Missouri, on Thursday, August 21 2014. The St. Louis suburb saw turmoil after a white police officer, Darren Wilson, fatally shot an unarmed black teenager, Michael Brown, on August 9, 2014. Some protesters and law enforcement officers clashed in the streets, leading to injuries and arrests. Hide Caption 1 of 76 Photos: Photos: Emotions run high in Ferguson A crowd gathers in Ferguson on August 21, 2014. With the situation appearing to calm, Gov. Jay Nixon ordered the Missouri National Guard to begin withdrawing from the city. Hide Caption 2 of 76 Photos: Photos: Emotions run high in Ferguson Police escort a Wilson supporter to a police vehicle, away from crowds protesting Brown's death on Wednesday, August 20, 2014. Police have said Brown and Wilson struggled over the officer's gun; some witnesses said Brown had his hands in the air when he was shot. Hide Caption 3 of 76 Photos: Photos: Emotions run high in Ferguson Lightning streaks over protesters on August 20, 2014. Hide Caption 4 of 76 Photos: Photos: Emotions run high in Ferguson Demonstrators protest on August 20, 2014. Hide Caption 5 of 76 Photos: Photos: Emotions run high in Ferguson U.S. Attorney General Eric Holder meets with Capt. Ron Johnson of the Missouri State Highway Patrol at a Ferguson restaurant on August 20, 2014. Holder came to Missouri to talk to community leaders and review the federal civil rights investigation into Brown's shooting. Hide Caption 6 of 76 Photos: Photos: Emotions run high in Ferguson Protesters march through the streets of Ferguson on August 20, 2014. Hide Caption 7 of 76 Photos: Photos: Emotions run high in Ferguson Police point out a demonstrator who has his arms raised before moving in to arrest him Tuesday, August 19, 2014. Hide Caption 8 of 76 Photos: Photos: Emotions run high in Ferguson People watch from inside a restaurant as protesters rally August 19, 2014. Hide Caption 9 of 76 Photos: Photos: Emotions run high in Ferguson Police arrest a demonstrator on August 19, 2014. Hide Caption 10 of 76 Photos: Photos: Emotions run high in Ferguson Police charge into the media work area with weapons drawn as they try to control demonstrators on August 19, 2014. Hide Caption 11 of 76 Photos: Photos: Emotions run high in Ferguson A protester speaks to a police officer on August 19, 2014. Hide Caption 12 of 76 Photos: Photos: Emotions run high in Ferguson People on August 19 stand near a memorial where Brown was shot and killed, 2014. Hide Caption 13 of 76 Photos: Photos: Emotions run high in Ferguson Protesters march on August 19, 2014. Hide Caption 14 of 76 Photos: Photos: Emotions run high in Ferguson Police watch as protesters march August 19, 2014. Hide Caption 15 of 76 Photos: Photos: Emotions run high in Ferguson A demonstrator is arrested on August 19, 2014. Hide Caption 16 of 76 Photos: Photos: Emotions run high in Ferguson Police try to control protesters on Monday, August 18, 2014. Hide Caption 17 of 76 Photos: Photos: Emotions run high in Ferguson Police lead a man away during a protest August 18, 2014. Hide Caption 18 of 76 Photos: Photos: Emotions run high in Ferguson A demonstrator shouts during a protest on West Florissant Avenue, one of Ferguson's main streets, on August 18, 2014. Hide Caption 19 of 76 Photos: Photos: Emotions run high in Ferguson Capt. Ron Johnson of the Missouri State Highway Patrol pulls his men back from aggressive protesters on August 18, 2014. Hide Caption 20 of 76 Photos: Photos: Emotions run high in Ferguson Police arrest a demonstrator August 18, 2014, after peaceful protests gave way to angry confrontations with authorities. Hide Caption 21 of 76 Photos: Photos: Emotions run high in Ferguson Law enforcement officers stand guard during a protest on West Florissant Avenue on August 18, 2014. Hide Caption 22 of 76 Photos: Photos: Emotions run high in Ferguson A woman helps a man affected by tear gas August 18, 2014. The situation overnight deteriorated after a handful of protesters threw rocks, bottles and Molotov cocktails at police. Officers responded by firing stun grenades and tear gas canisters. Hide Caption 23 of 76 Photos: Photos: Emotions run high in Ferguson Officers stand with weapons drawn during a protest on West Florissant Avenue on August 18, 2014. Hide Caption 24 of 76 Photos: Photos: Emotions run high in Ferguson Musician Nelly, center, joins demonstrators in Ferguson on August 18, 2014. Hide Caption 25 of 76 Photos: Photos: Emotions run high in Ferguson Demonstrators receive red roses as they protest August 18, 2014. Hide Caption 26 of 76 Photos: Photos: Emotions run high in Ferguson Demonstrators march on August 18, 2014. Hide Caption 27 of 76 Photos: Photos: Emotions run high in Ferguson The Rev. Jesse Jackson shakes hands with a police officer as he visits Ferguson's demonstration area on August 18, 2014. Hide Caption 28 of 76 Photos: Photos: Emotions run high in Ferguson A protester picks up a tear gas canister on Sunday, August 17, 2014. Hide Caption 29 of 76 Photos: Photos: Emotions run high in Ferguson Police wait to advance after using tear gas to disperse protesters August 17, 2014. Hide Caption 30 of 76 Photos: Photos: Emotions run high in Ferguson Water gets poured into a woman's eyes after a tear gas attack by police on August 17, 2014. Hide Caption 31 of 76 Photos: Photos: Emotions run high in Ferguson A man runs through clouds of tear gas on August 17, 2014. Hide Caption 32 of 76 Photos: Photos: Emotions run high in Ferguson Protesters react as police fire tear gas at them August 17, 2014. Hide Caption 33 of 76 Photos: Photos: Emotions run high in Ferguson Police advance through a cloud of tear gas on August 17, 2014. Most of the crowd had dispersed after a curfew went into effect at midnight, St. Louis County authorities said. Hide Caption 34 of 76 Photos: Photos: Emotions run high in Ferguson People take cover from tear gas inside a McDonald's on August 17, 2014. Hide Caption 35 of 76 Photos: Photos: Emotions run high in Ferguson A large group of police officers advance toward protesters on August 17, 2014. Hide Caption 36 of 76 Photos: Photos: Emotions run high in Ferguson A man fights the effects of tear gas in Ferguson on August 17, 2014. Hide Caption 37 of 76 Photos: Photos: Emotions run high in Ferguson Michael Brown Sr. and Lesley McSpadden, the parents of Michael Brown, attend a rally at Greater Grace Church in Ferguson on August 17, 2014. Hide Caption 38 of 76 Photos: Photos: Emotions run high in Ferguson The Rev. Al Sharpton hugs McSpadden during the rally. Hide Caption 39 of 76 Photos: Photos: Emotions run high in Ferguson Capt. Ron Johnson of the Missouri State Highway Patrol speaks at the rally. He had been appointed by the governor to take control of security operations. Hide Caption 40 of 76 Photos: Photos: Emotions run high in Ferguson Demonstrators defy a curfew early on August 17, 2014. Hide Caption 41 of 76 Photos: Photos: Emotions run high in Ferguson Police fire tear gas at demonstrators after curfew on August 17, 2014. Hide Caption 42 of 76 Photos: Photos: Emotions run high in Ferguson A law enforcement officer aims his rifle August 17,, 2014, after tear gas was fired to disperse a crowd. Hide Caption 43 of 76 Photos: Photos: Emotions run high in Ferguson Law enforcement officers check a building on August 17, 2014. Hide Caption 44 of 76 Photos: Photos: Emotions run high in Ferguson People attend a protest after the midnight curfew on August 17, 2014. Hide Caption 45 of 76 Photos: Photos: Emotions run high in Ferguson A law enforcement officer watches as tear gas is fired to disperse a crowd on August 17, 2014. Hide Caption 46 of 76 Photos: Photos: Emotions run high in Ferguson Police stand guard before the midnight curfew on Saturday, August 16, 2014. Hide Caption 47 of 76 Photos: Photos: Emotions run high in Ferguson People loot the Ferguson Market and Liquor store on August 16, 2014. Several businesses were looted as police held their positions nearby. Hide Caption 48 of 76 Photos: Photos: Emotions run high in Ferguson Missouri State Highway Patrol officers listen to taunts from demonstrators during a protest on Friday, August 15, 2014. Hide Caption 49 of 76 Photos: Photos: Emotions run high in Ferguson Demonstrators protest with their hands up on August 15, 2014. The "hands up" gesture has become a symbol in protests as Brown, according to eyewitnesses, was trying to surrender when he was shot multiple times. Hide Caption 50 of 76 Photos: Photos: Emotions run high in Ferguson Police confront demonstrators on August 15, 2014. Hide Caption 51 of 76 Photos: Photos: Emotions run high in Ferguson A demonstrator throws a tear gas canister back at police on August 15, 2014. Hide Caption 52 of 76 Photos: Photos: Emotions run high in Ferguson Police confront demonstrators on August 15, 2014. Hide Caption 53 of 76 Photos: Photos: Emotions run high in Ferguson ri – Thousands of demonstrators march down a Ferguson street with members of the St. Louis County Police and the Missouri State Highway Patrol on Thursday, August 14, 2014. Hide Caption 54 of 76 Photos: Photos: Emotions run high in Ferguson Demonstrators hold signs as traffic moves slowly past them on August 14, 2014. Hide Caption 55 of 76 Photos: Photos: Emotions run high in Ferguson People scramble as police fire tear gas on August 17, 2014. Hide Caption 56 of 76 Photos: Photos: Emotions run high in Ferguson State Highway Patrol Capt. Ron Johnson smiles at demonstrators on August 14, 2014. Johnson was appointed to lead security as state troopers took over after days of clashes between protesters and local police. Hide Caption 57 of 76 Photos: Photos: Emotions run high in Ferguson The Rev. Traci Blackmon uses a megaphone to talk to a large group of demonstrators on August 14, 2014. Hide Caption 58 of 76 Photos: Photos: Emotions run high in Ferguson A man picks up a flaming bottle and prepares to throw it as a line of police advance in the distance on Wednesday, August 13, 2014. Hide Caption 59 of 76 Photos: Photos: Emotions run high in Ferguson Police stand in clouds of smoke as they clash with protesters on August 13, 2014. Hide Caption 60 of 76 Photos: Photos: Emotions run high in Ferguson An Al-Jazeera television crew runs for cover as police fire tear gas at its position on August 13, 2014. Video and images on social media showed police later breaking down the journalists' gear. Hide Caption 61 of 76 Photos: Photos: Emotions run high in Ferguson A protester runs from tear gas exploding around him on August 13, 2014. Hide Caption 62 of 76 Photos: Photos: Emotions run high in Ferguson Police stand guard among demonstrators on August 13, 2014 Hide Caption 63 of 76 Photos: Photos: Emotions run high in Ferguson Ferguson Police Chief Thomas Jackson fields questions during a news conference on August 13, 2014. Hide Caption 64 of 76 Photos: Photos: Emotions run high in Ferguson A small group of protesters block traffic in the street before police arrived on August 13, 2014. Hide Caption 65 of 76 Photos: Photos: Emotions run high in Ferguson A protester throws a tear gas canister back toward police on August 13, 2014. Hide Caption 66 of 76 Photos: Photos: Emotions run high in Ferguson Police detain a man on Tuesday, August 12, 2014. Hide Caption 67 of 76 Photos: Photos: Emotions run high in Ferguson People congregate at the Greater St. Marks Family Church in St. Louis along with the family of Michael Brown and the Rev. Al Sharpton on August 12, 2014. Hide Caption 68 of 76 Photos: Photos: Emotions run high in Ferguson Michael Brown Sr. stands alongside Sharpton, right, during a news conference in St. Louis on August 12, 2014. Hide Caption 69 of 76 Photos: Photos: Emotions run high in Ferguson Demonstrators protest August 12 in Ferguson, 2014. Hide Caption 70 of 76 Photos: Photos: Emotions run high in Ferguson A makeshift memorial sits in the middle of the street where Michael Brown was shot and killed. Hide Caption 71 of 76 Photos: Photos: Emotions run high in Ferguson A woman tries to calm an emotional protester during a demonstration outside the headquarters of the Ferguson Police Department on August 11, 2014. Hide Caption 72 of 76 Photos: Photos: Emotions run high in Ferguson Phaedra Parks, left, comforts Desuirea Harris, the grandmother of Michael Brown, during a news conference in Jennings, Missouri, on August 11, 2014. Hide Caption 73 of 76 Photos: Photos: Emotions run high in Ferguson Police officers arrest a man who refused to leave when police cleared streets in Ferguson on August 11, 2014. Hide Caption 74 of 76 Photos: Photos: Emotions run high in Ferguson A burned-out QuikTrip gas station smolders on August 11, 2014 after protesters looted and burned the Ferguson building the night before. Hide Caption 75 of 76 Photos: Photos: Emotions run high in Ferguson Police officers and protesters confront each other on Saturday, August 9, the same day Michael Brown was shot and killed. Hide Caption 76 of 76

"Far more people are being killed in America's cities this year than in many years -- and let's be clear: far more people of color are being killed in America's cities this year. And it's not the cops doing the killing," Comey said.

The FBI chief repeatedly used the phrase "all lives matter" in various contexts during his discussion led by Ruby Garrett, editor of the law school's Legal Forum and president of the Black Law Students Association. The phrase has drawn controversy because some view it as a response to the advent of the Black Lives Matter movement. The two spoke before a mostly white student audience.

The provocative remarks expanded on themes Comey first broached in a 2014 speech at Georgetown University, where he acknowledged racial biases were at times to blame for "lazy mental shortcuts" that lead to more police stops of young black men.

But in Friday's speech, and again Sunday, he said he was trying to start a conversation about whether the pendulum has swung too far.

"In today's YouTube world, are officers reluctant to get out of their cars and do the work that controls violent crime? Are officers answering 911 calls but avoiding the informal contact that keeps bad guys from standing around, especially with guns?" he asked in his Friday remarks. "I don't know whether this explains it entirely, but I do have a strong sense that some part of the explanation is a chill wind blowing through American law enforcement over the last year. And that wind is surely changing behavior."

Ray Kelly, the former commissioner of the New York Police Department, said Monday that police are no longer "taking the initiative," which he said accounts for some of the rise in crime.

"I commend Jim Comey for telling it like it is," Kelly told CNN's Wolf Blitzer on "The Situation Room." "Officers are not engaging in proactive policing, not engaging in the levels they engaged in the recent past."

White House press secretary Josh Earnest said Monday there was no evidence that police officers were "shirking" their duties given increased scrutiny on law enforcement, seeming to rebut FBI Director James Comey's assertion last week.

"The available evidence at this point does not support the notion that law enforcement officers are shying away from fulfilling their responsibilities," Earnest said at the daily briefing.

He cited national law enforcement leaders saying that there's little proof that police forces are relaxing their practices after high profile incidents of videotaped police brutality.

"The evidence we've seen so far doesn't support the contention that law enforcement officials are somehow shirking their responsibility, and in fact you've seen law enforcement leaders across the country indicating that's not what's taking place," he said.

Comey appeared to soften his tone in his remarks in Chicago on Monday to International Association of Chiefs of Police convention in Chicago.

Comey noted the discordant tones of the social media hashtags #BlackLivesMatter and #PoliceLivesMatter, which he said was pitting police against critics who believe black communities are bearing the brunt of heavy-handed police tactics.

This, Comey said, "is a terrible place for us to be."

He urged police chiefs to help the FBI gather better data about of officer-involved shootings and other use-of-force incidents.

"We need to figure out what's happening," Comey said, adding that he also wants data on assaults on cops.

Photos: Unrest in Ferguson Photos: Unrest in Ferguson Police officers walk past the smoldering remains of a beauty supply store in Ferguson, Missouri, on Tuesday, November 25. Ferguson has been struggling to return to normal since Michael Brown, an unarmed black teenager, was killed by Darren Wilson, a white police officer, on August 9. The grand jury did not indict Wilson in the case, prompting new waves of protests in Ferguson and across the country. Hide Caption 1 of 34 Photos: Unrest in Ferguson A woman cleans up glass from a business' shattered window on November 25. Hide Caption 2 of 34 Photos: Unrest in Ferguson A Ferguson firefighter surveys rubble at a strip mall that was set on fire overnight. Hide Caption 3 of 34 Photos: Unrest in Ferguson Protesters run away after police deployed tear gas in Ferguson on Monday, November 24. Hide Caption 4 of 34 Photos: Unrest in Ferguson Police take position during clashes with protesters on November 24. Hide Caption 5 of 34 Photos: Unrest in Ferguson A protester stands in front of police vehicles with his hands up on November 24. Hide Caption 6 of 34 Photos: Unrest in Ferguson Police in riot gear move past a burning vehicle on November 24. Hide Caption 7 of 34 Photos: Unrest in Ferguson A looter in Ferguson walks out of a burning Walgreens on November 24. Hide Caption 8 of 34 Photos: Unrest in Ferguson Riot police clash with protesters on November 24. Hide Caption 9 of 34 Photos: Unrest in Ferguson Firefighters work on extinguishing a Little Caesars restaurant on November 24. Hide Caption 10 of 34 Photos: Unrest in Ferguson Smoke fills the streets of Ferguson as buildings burn on November 24. Hide Caption 11 of 34 Photos: Unrest in Ferguson Police officers grab a protester on November 24. Hide Caption 12 of 34 Photos: Unrest in Ferguson A woman treats her face for possible tear gas exposure on November 24. Hide Caption 13 of 34 Photos: Unrest in Ferguson People walk away from a burning storage facility on November 24. Hide Caption 14 of 34 Photos: Unrest in Ferguson A man steps out of a vandalized store on November 24. Hide Caption 15 of 34 Photos: Unrest in Ferguson A police officer runs by a burning police car on November 24. Hide Caption 16 of 34 Photos: Unrest in Ferguson Police officers stand guard as protesters confront them on November 24. Hide Caption 17 of 34 Photos: Unrest in Ferguson Protesters block streets in St. Louis after the announcement of the grand jury's decision on November 24. Ferguson is a suburb of St. Louis. Hide Caption 18 of 34 Photos: Unrest in Ferguson Police confront protesters in Ferguson on November 24. Hide Caption 19 of 34 Photos: Unrest in Ferguson A police officer points his rifle at demonstrators on November 24. Hide Caption 20 of 34 Photos: Unrest in Ferguson Protesters run for shelter as smoke fills the streets of Ferguson on November 24. Hide Caption 21 of 34 Photos: Unrest in Ferguson The glass windows of a store are shattered on November 24. Hide Caption 22 of 34 Photos: Unrest in Ferguson A demonstrator listens to a car radio as the grand jury's decision is delivered in front of the Ferguson Police Department. Hide Caption 23 of 34 Photos: Unrest in Ferguson Lesley McSpadden, mother of Michael Brown, is escorted away from the Ferguson Police Department on November 24. Hide Caption 24 of 34 Photos: Unrest in Ferguson A group of protesters vandalizes a police vehicle in Ferguson on November 24. Hide Caption 25 of 34 Photos: Unrest in Ferguson Police officers confront protesters on November 24. Hide Caption 26 of 34 Photos: Unrest in Ferguson Demonstrators block traffic during a protest in front of the Ferguson Police Department on November 24. Hide Caption 27 of 34 Photos: Unrest in Ferguson Demonstrators gather outside the police station on November 24. Hide Caption 28 of 34 Photos: Unrest in Ferguson Protesters gather as they wait for the announcement of the grand jury decision on November 24. Hide Caption 29 of 34 Photos: Unrest in Ferguson Members of the media line up in a parking lot across from the Buzz Westfall Justice Center on November 24. Hide Caption 30 of 34 Photos: Unrest in Ferguson Residents begin to gather at the Michael Brown memorial ahead of the grand jury announcement. Hide Caption 31 of 34 Photos: Unrest in Ferguson National Guard troops arrive ahead of the grand jury announcement. Hide Caption 32 of 34 Photos: Unrest in Ferguson Members of the Missouri National Guard are escorted out of the Buzz Westfall Justice Center. Hide Caption 33 of 34 Photos: Unrest in Ferguson Demonstrators are confronted by police as they block a street before the grand jury announcement. Hide Caption 34 of 34

To be sure, violent crime remains at historic lows.

Even in cities where dozens of shootings in a weekend regularly make national headlines, violent crime rates were far higher 20 years ago. Chicago has recorded over 400 murders this year, but that's well short of the more than 900 common in the early 1990s. Police in Washington, D.C., which recorded nearly 400 murders in 1996, are facing public pressure because of 124 killings this year, an increase from 88 in 2012.

Comey's cautious endorsement of the Ferguson Effect may be welcomed by some in policing, but many other police chiefs disagree. To do so, some say privately, would admit that their officers aren't doing their jobs. Many other chiefs blame a rise in synthetic drugs and other discrete causes for crime.

New York City Police Commissioner William Bratton says he's not seeing the so-called Ferguson effect in his city. There was a brief period last winter, in the wake of the ambush murder of two officers, that arrests dipped before returning to normal levels.

In an interview with CNN, Bratton said he believes what's happening around the country is part of the "evolution of policing" which will eventually make officers better.

"A cop sitting in his car is not what a cop should be," Bratton said.

Groups that grew out of the Ferguson protests dispute that crime rates have any link to their efforts to shine a light on what they believe is widespread problem of excessive use of force.

The nature of U.S. policing, with more than 12,000 local police departments, makes it difficult to gather statistics on police shootings. Because not all police departments collect and report such numbers, no national count exists.

The president and attorney general are both expected to their pitch for criminal justice reform to the gathering of police.

But both are likely to give remarks generally supportive of police.

That's a message that likely will go over well inside the convention hall in Chicago. Outside, the jury remains out.