CHICAGO – Chicago police solved fewer than one in six homicides committed in the city in the first half of 2018, continuing a troubling decline in the number of perpetrators being brought to justice in one of the nation’s most violent cities, data obtained by USA TODAY shows.

Chicago's homicide clearance rate – the percentage of cases in which police arrest or identify a suspect – fell from 17.1 in 2017 to 15.4 during the first six months of 2018, the data shows. If that rate holds through the end of the year, it would be the sixth consecutive annual decline.

Police in the nation's third-largest city are having even less success solving nonfatal shootings, according to the data obtained through a Freedom of Information Act request. Police cleared 50 of 900 nonfatal shooting incidents in the first half of 2018, a rate of 5.6 percent.

That puts them on pace to solve fewer than 9 percent of nonfatal shooting incidents for the fourth year in a row.

Top officials in the department of 13,500 sworn officers say the clearance rate reflects an alarming dynamic in the violent neighborhoods of Chicago's West and South sides: Shooting victims often forego cooperation with authorities to seek retaliation on their own.

That fuels more violence, police say, and further erodes trust in law enforcement.

Deputy Chief Brendan Deenihan heads the Chicago Police Department’s detective division. He says the victims and offenders are often “interchangeable.”

“These are guys who are shooting back and forth at each other on a consistent basis,” Deenihan told USA TODAY. “They're not afraid to go to court and testify. They just want to get even with the people who shot at them.”

Falling clearance rates are not just a Chicago problem. The national clearance rate for homicides fell to 59.4 percent in 2016, the lowest since the FBI began tracking them in 1965.

In Indianapolis, where the murder rate has surged, the Metropolitan Police command staff have called on outside experts to help with the growing number of unsolved homicides.

The Midwest city has seen its clearance rate tumble from 66 percent in 2014 to 40 percent last year, according to an analysis by the Indianapolis Star, a member of the USA TODAY Network.

Phoenix saw its homicide clearance rate tumble from 90 percent in 2013 to 57 percent last year. The Phoenix Police Department saw its clearance rate improve several years ago after winning a federal National Institute of Justice grant, but then decline after the grant ran out.

The department is hoping to add more detectives soon to bolster its cold case unit. The clearance rate has climbed to 62.3 percent in the first half of 2018.

“There hasn't been a significant change in working with witnesses and the community over the years to solve these cases,” Phoenix Police Sgt. Mercedes Fortune said. “We understand the importance of our community partnerships and continue to foster that relationship.”

Clearance rates have been in the spotlight in Chicago, which suffered 650 homicides last year and 762 homicides in 2016, more than any other city in the nation. President Donald Trump has repeatedly lashed out against city leaders over their handling of the violence.

The rates have become an issue in the crowded race to replace Mayor Rahm Emanuel, who announced this month he would not run for a third term in the city’s February election.

One contender, former Chicago Public Schools CEO Paul Vallas, has called for increasing the department’s detective pool from 1,000 to 1,200.

Homicides are down about 19 percent so far in 2018 compared to the same point last year. The city remains on pace for more than 500 killings for the fourth consecutive year.

After a particularly violent weekend last month in which more than 70 people were shot, Emanuel and Police Superintendent Eddie Johnson decried the lack of cooperation from residents in the predominantly low-income, black neighborhoods that suffer most of the city's violence.

“You all know who these individuals are. They come into your homes every day, sleep with you every night,” Johnson said. “Grandparents, parents, siblings, significant others – you know who they are.”

Seventy-two people were shot during the early August weekend, 12 of them fatally. Police have made only two arrests.

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Deenihan described a shooting last month allegedly by a repeat gun offender.

A 26-year-old man was shot in the foot on the city's West Side. The department’s gunfire-sensing ShotSpotter system helped narrow down the location of the shooter. Police used surveillance cameras to spot a car leaving the area and tracked the vehicle until patrol officers were able to catch up to it.

Police say two men in the car pulled into a gas station. They say workers saw 27-year-old Rick Franklin drop what turned out to be a stolen gun near a doughnut display inside the gas station store.

But the victim of the shooting refused to cooperate, Deenihan said, so police were able to charge Franklin only with unlawful use of a weapon. Franklin was prohibited from possessing a weapon due to his criminal history.

Illinois sentencing guidelines for assault with a deadly weapon call for up to 10 years in prison. The lesser weapons charge carries a maximum of three years.

“We had technology and good police work,” Deenihan said. “But you have a victim who was shot that doesn’t want to cooperate, and you have the individual who did the shooting – a repeat gun offender – who is eventually going to be out on the street again.”

Deenihan said detectives are used to victims refusing to cooperate.

Romell Young, 23, says he knows who shot him in April near his home on the West Side. He told USA TODAY that he didn't cooperate with the police investigation because it would have escalated the spat, made him vulnerable to retaliation and sullied his reputation.

“It could have been bad for me because it would (ruin) my name,” Young said.

The falling clearance rates coincide with the declining trust in the police department.

Long-strained relations between police and the city's African-American community have deteriorated further since the 2015 release of a video that shows the police shooting of a black teen.

Police say 17-year-old Laquan McDonald was wielding a retractable knife with a 3-inch blade. The video appears to show that McDonald had turned away from police when Officer Jason Van Dyke opened fire.

Van Dyke is now on trial for first-degree murder, aggravated battery and official misconduct.

“For our detectives, it’s a grind out there," Deenihan said. "They go out to the scenes. They do their best.

“We do live in a society today where people don’t cooperate with the police.”

Even before the McDonald shooting, the police department’s relationship in the African-American community had been strained by a long history of police brutality and allegations of heavy-handed tactics in some of the same neighborhoods most impacted by violence.

The Rev. Ira Acree, a pastor and activist on the West Side, said blaming residents in violence-plagued neighborhoods for low clearance is “ludicrous” and “offensive.”

Acree notes that Chicago police’s relationship with the African-American community has been strained for decades. During the crack cocaine epidemic of the 1990s, conflicts between gangs drove homicide rates higher than those of today.

Still, Chicago detectives annually cleared more than 60 percent of killings, according to the FBI.

“At some point, the police department has to take some responsibility,” Acree said.

Contributing: Jason Pohl of the Arizona Republic