The following is an excerpt from Handcuffed: What Holds Policing Back and the Keys to Reform.

These are tumultuous times for policing in America. Thanks in part to the almost ubiquitous presence of video cameras, the American public has recently had the chance to see the very best and the very worst of police conduct.

Thousands of police officers attend the memorial service for Massachusetts Institute of Technology (MIT) police officer Sean Collier at MIT in Cambridge, Massachusetts April 24, 2013. REUTERS/Brian Snyder

At the scene of the Boston Marathon bombings on April 15, 2013, Boston police officers and other emergency workers instinctively ran toward the site of the explosions to help the injured and take control of the scene, even while nobody knew how many more bombs there might be. Video footage made plain to all the classic courage of first responders reacting to a traumatic situation with professional discipline and putting their own lives at risk for the sake of the public they serve.

Three days later, on April 18, MIT patrol officer Sean Collier was shot dead in his patrol car by bombing suspects Dzhokhar and Tamerlan Tsarnaev, who were apparently seeking to acquire weapons and perhaps provoke a major confrontation with police. In an extraordinary display of public appreciation for police officers and the dangers they face on a daily basis, more than 10,000 people attended Officer Collier’s funeral.

“Citizens of any mature democracy can expect and should demand police services that are responsive to their needs, tolerant of diversity, and skillful in unraveling and tackling crime and other community problems.”

Scores of law enforcement officers from federal, state, and local agencies had flooded into the area and cooperated in the search. When it was all over, local residents — who had voluntarily heeded the police request to “shelter in place” — emerged from their homes, gathered on street corners, and spontaneously applauded as buses full of law enforcement officers passed by.

During that week in April 2013, nobody seemed to have anything but praise for the courageous and selfless way police conducted themselves in the face of those extraordinary dangers.

But 2014 and 2015 brought to public attention a series of incidents, many of them video-recorded on the cellphones of passersby, that appalled the public, astonished many, and raised troubling questions about the quality and nature of policing in America. Several incidents involving the deaths of unarmed black men at the hands of white police officers, albeit in different jurisdictions, came in quick enough succession to be perceived as a pattern and to prompt national debate. The pattern was pretty much established after two high profile incidents just three weeks apart: the death of Michael Brown in Ferguson, Missouri, and of Eric Garner in New York City. Public concern over the issues raised drew commentary from the president, led to the establishment of a presidential task force, resulted in investigations of patterns of police conduct by the Department of Justice’s Civil Rights Division, and spawned protests against police violence — particularly against minorities — that spread across the nation far beyond the cities directly involved.