A group of suspected gang members assembled on a north Minneapolis block earlier this year to mourn a slain friend when several gunmen walked up and started shooting.

The men on the block fired back, scattering spent .45-caliber casings on the pavement. Sixty rounds later, a 20-year-old named Roy Davis lay dead in the street and seven others were wounded.

No one has been charged in the May 4 shooting. Police heard whispers on the streets that a well-known member of a rival gang had been involved, but the homicide detectives assigned to the case didn't have any eyewitnesses who could place him at the scene.

The incident is one example of how more killings in Minneapolis are going unsolved this year, frustrating police detectives and chipping away at residents' sense of security.

So far this year, 28 people have been slain in the city, which is on pace to have fewer homicides than the 50 recorded in 2015. Despite this decrease, the department has made arrests in less than half of those cases, the lowest clearance rate in at least 12 years.

Criminal charges have been filed in only 13 of the 27 homicide investigations so far this year (one was a double slaying), city records show.

In 2015, Minneapolis police solved more than half of the city's slayings, clearing 26 of 45 homicide cases.

Minneapolis police officials say that clearance rates present simply a snapshot in time that doesn't fully capture the homicide unit's success.

"It's not like at the end of the year we take all the 2016 cases and put them on a shelf and they're done," said Deputy Chief Bruce Folkens. He said one factor could be a decline in domestic homicides, where the suspect is often still at the scene when police arrive.

He said he expects the department's clearance rate to improve after medical examiners determine the cause of death in a recent suspected murder-suicide.

But several recent cases have left investigators stumped.

After Birdell Beeks, a 58-year-old grandmother, was killed by stray gunfire on the North Side in May, police questioned several youths who were seen running away into a nearby house. The shooting became a rallying cry to end violence and led to a temporary cease-fire between battling gangs. But no one has been charged. A $10,000 reward for any information has gone unclaimed.

Most unsolved on North Side

Among the city's five police precincts, the largest number of unsolved homicides, 13, was recorded in the Fourth Precinct, which covers the neighborhoods north of downtown and west of Interstate 94. Arrests were made in only six slayings in that precinct this year.

After 2-year-old Le'Vonte King Jason Jones was killed in July by someone who shot into the van he was riding in, police made an arrest, but prosecutors determined there was inadequate evidence to charge the suspect.

Jones' father, who police say shot at the gunman, was charged with murder, meaning the case is considered cleared.

Police also arrested two men in connection with the March 26 death of 18-year-old Derrick Mack in the North Side's Webber-Camden neighborhood, but neither has been charged.

The department's clearance rate for murder peaked at 79 percent in 2006, when there were 61 homicides. The 10-year clearance average is about 66 percent.

While Minneapolis has bucked a national trend that has seen violent crimes rise in many large cities, like other departments it has struggled to solve its killings.

Nationally, the percentage of homicides solved last year was about 64 percent, criminologists say.

Detectives in Dallas, which also has seen its clearance rates fall, have solved 40 percent of their cases in 2016, while their counterparts in Oakland, Calif., and Milwaukee clear about 60 percent.

In Chicago, where homicides are piling up at a staggering pace, police leaders promised to hire hundreds of new detectives to help improve the department's clearance rate of 31 percent.

Criminologists consider tougher prosecutorial standards for charging murders and strained police-public relationships as possible explanations for the declining clearance rates. But they also caution against reading too much into year-over-year comparisons.

"That lack of trust leads to you can't get witnesses to come forward, you can't get witnesses to testify in court," said Steve Lurie, an adjunct professor at Loyola Law School and a 20-year law enforcement veteran.

In some cases, local police say, they are only an anonymous tip or confession away from making a breakthrough.

Detectives investigating the death of Crystal Collins, a 36-year-old mother of three killed in a drive-by shooting this summer, concluded that someone in the group she was hanging out with returned fire. Some of the shell casings at the scene matched cartridges that were found at another homicide, at a parking lot gathering on July 3, police said. But so far, detectives haven't determined who pulled the trigger in either case.

The department has tried a variety of strategies to coax reluctant witnesses to come forward, including the creation of its crisis response team of respected community leaders who show up at violent crime scenes to offer counseling. Police also direct witnesses to the anonymous tip line if they are wary of giving their names for fear of retaliation or being branded a "snitch."

Earlier this year, city officials announced another program in which social workers intervene with gunshot victims at Hennepin County Medical Center to ward off further violence.

Department officials touted recent successes in solving homicides, pointing out that suspects have been charged in eight of the 10 most recent slayings.

In the latest case, 20-year-old Gustav Christianson and an infant, Jayden Redden, were shot dead in a parked car. Less than 48 hours later, police arrested 17-year-old Jquan McInnis, who they believe was angry over an old debt.

Cmdr. Erick Fors, who runs the Violent Crimes Investigations Division, attributed the arrest to a combination of witness cooperation and old-fashioned detective work.

"Even though technology increases, it always comes down to communication, to building that trust, and I think that's something that our detectives are very good at," Fors said.

Libor Jany • 612-673-4064

Twitter: @StribJany