Minneapolis police are bringing in outside help as they try to temper violence that killed four people this weekend alone, including a college senior who was out celebrating graduation.

Mayor Jacob Frey said the city has asked state and federal agencies for assistance, citing the city's shortage of officers.

"Safety in our city has to be a priority," Frey said at a news conference Sunday, calling the reinforcements "really, really critical."

The weekend's victims include two men believed to have been struck by gunfire on the North Side and two men killed in a mass downtown shooting: one of the suspected gunmen and the student, whose family said was "an innocent bystander."

The increase in violence has tested the commitment of city leaders who unanimously promised to transform policing and public safety in the wake of George Floyd's death — but are deeply divided about how to proceed.

While debates about policing have proved to be divisive within City Hall, some of the victims' relatives said they hope to remove politics from the discussion. They want a solution that will keep other families from feeling pain like theirs.

University of St. Thomas student Charlie Johnson went out with friends on Friday night to celebrate their graduation the next day.

Just before 2 a.m. Saturday, gunfire erupted along N. 1st Avenue between 3rd and 4th streets downtown.

Johnson and his friends were walking home when they got hit by stray gunfire, his father, Greg Johnson, said Sunday. A close friend stayed nearby and tried to provide "aid and comfort."

"He was a ray of sunshine without a mean bone in his body," Greg Johnson said of his son.

On Facebook, he described Charlie as "the best of all of us," saying he "left a beautiful impact on every single life he touched."

Greg Johnson asked people to say his son's name and to work toward ending gun violence.

"No politics please," he wrote, "just love and compassion for each other as Charlie had for all of us. I AM CHARLIE!"

Police said shots erupted at bar closing time when two men got into a "verbal confrontation" on the crowded sidewalk. In addition to the two killed, eight people were injured by the gunfire.

The other suspected shooter, a Bloomington man, 23, was arrested and booked on suspicion of murder. He had not been charged as of Sunday evening. The Star Tribune generally does not name suspects before they have been charged.

Hours before the mass shooting, police responding to a ShotSpotter activation in the city's Jordan neighborhood about 8:40 p.m. found a car-crash victim with gunshot wounds. The man, whose name has not been released, died a short time later.

That same night, there was a nonfatal shooting at the CC Club in south Minneapolis.

Sunday, too, brought another death. Police suspect a man, whose name has not been released, was shot in the city's Hawthorne neighborhood. As people tried to rush him to the hospital to get treatment for his wounds, the car flipped, police said.

The Medical Examiner's Office will rule on the precise cause of his death.

Excluding that case, the city has had 31 homicides so far this year.

The weekend's carnage added another layer of trauma to a community reeling from the shooting of three children on Minneapolis' North Side.

On Sunday afternoon, relatives of the children gathered outside City Hall for an emotional news conference announcing a reward for information leading to arrests and convictions in their cases.

Amid their pleas for information, they also called for peace in the community.

The group has been keeping a vigil outside North Memorial Health Hospital. On Friday night, they heard the "pop, pop, pop" of gunshots coming from the direction of Broadway Avenue, said Randy Ottoson. He's the grandfather of Trinity Ottoson-Smith, 9, who was shot this month while jumping on a trampoline in a north Minneapolis yard.

"We need more police officers. There is no doubt in my mind," he said. Ottoson said the city needs need police reform because Black lives matter, but he believes it needs more police on the street, too.

"I'm praying for all of these families but, you know what, I'm praying for the next people too," Ottoson said.

The surge in violence comes as the city is grappling with the coronavirus pandemic and the aftermath of Floyd's killing.

Nearly 200 Minneapolis police officers have left the department since Floyd's death, including dozens who filed PTSD claims after the unrest.

The issue of police staffing has been contentious within City Hall. Frey unveiled a plan to eventually build the force's ranks back up close to its prior levels.

Some other elected officials have argued the emphasis on police staffing is misplaced and that an increase in officers doesn't guarantee a decrease in violence.

Almost all of the city's elected leaders have spoken about the importance of expanding mental health and violence prevention services, but they differ on whether police funding should be used to support that.

Some of the possible solutions will be long-term — hinging on negotiations about how to use American Rescue Plan funding, or a proposal to replace MPD.

Chief Medaria Arradondo provided few specifics about the shooting investigations Sunday, saying he didn't want to jeopardize them.

Minneapolis police said they called in seven homicide detectives to work on the mass shooting case.

The city's Office of Violence Prevention sent a "very small group," including director Sasha Cotton, to work with families and bystanders, city spokeswoman Sarah McKenzie said.

They also sent "a few teams" to work downtown from roughly 10 p.m. Saturday to 3 a.m. Sunday "engaging and de-escalating conflicts," she said.

Frey said the city received help from state troopers Saturday night into Sunday and said the city is working with the state Bureau of Criminal Apprehension.

Arradondo said federal agencies also will help.

Victims' families deserve information, the chief said, imploring people to call in tips on all the cases, including on the children's shootings: "People have got to step up."