BALTIMORE -- Gunfire erupted at a block party in Baltimore on Sunday -- killing two people, wounding 28 and leaving an extensive crime scene that marred the U.S. holiday weekend, police said. Three of the wounded were in critical condition and more than a dozen were under 18.

The shooting took place just after 12:30 a.m. when at least two people opened fire at a block party in the Brooklyn Homes area in the southern part of the city, said Richard Worley, Baltimore's acting police commissioner. No arrests had been made by late afternoon. It wasn't clear if the shooting was targeted or random, Worley said.

The shooting comes amid gatherings around the country leading up to the July Fourth holiday. Elsewhere, a shooting in Kansas left seven people with gunshot wounds and two more victims hospitalized after being trampled as people rushed out of a nightclub early Sunday morning, police there said.

The violence in Baltimore occurred as federal prosecutors there this week touted their efforts to reduce violent crime in the city. Police have reported nearly 130 homicides and close to 300 shootings so far this year, though that's down from the same time last year. Authorities have vowed to crack down aggressively on repeat violent offenders.

Nine of Sunday's victims were taken by ambulance and 20 walked into area hospitals with injuries from the shooting, Worley said. Nine remained hospitalized Sunday afternoon.

The deceased victims were identified as 18-year-old Aaliyah Gonzales and 20-year-old Kylis Fagbemi, police said Sunday. Gonzales died at the scene, and Fagbemi died at the hospital. The 28 injured ranged in ages from 13 to 32, with more than half of them being younger than 18, officials said.

"I want those who are responsible to hear me, and hear me very clearly," Mayor Brandon Scott said at the scene. "We will not stop until we find you, and we will find you. Until then, I hope that every single breath you take, that you think about the lives that you took, think about the lives that you impacted here tonight."

Scott asked anyone with information to come forward to assist investigators locate the "cowards" who were responsible for the shooting.

Gov. Wes Moore said his "heart breaks for these victims, their families, and the Baltimore community that is coping with the loss."

"Maryland has had enough of watching gun violence continue to ravage our state and our nation," Moore said in a statement. "The fact that these horrific shootings continue to take place is abominable. We as a state will continue to do everything we can to prevent senseless acts of violence like the one we saw last night."

Authorities said the crime scene was extensive and that it took some time for detectives to work it.

Hours after the shooting, a number of officers remained working behind police tape amid densely packed two-story housing blocks. Folding tables and plastic cups were scattered on the street, apparently left behind when people ran from the gunshots.

Lakell Nelson said there had been several false alarms of people mistaking the sounds of fireworks for gunfire earlier in the night while she was at the block party. The actual shooting started as she was getting to her car.

"The shots were just going on and on and on," she said.

That's when two young women approached her and said they had been shot, with one woman showing how a bullet had gone through her shorts.

Nelson said she told the women to get in the car, and she sped through red lights to get to the nearest hospital.

"When I pulled up to the door of the hospital, my car was almost getting ready to be inside the hospital, because I was determined to get those babies in that hospital," Nelson said.

A police union official said in an email Sunday there were no officers specifically assigned to the gathering.

"There were only three officers assigned to the Brooklyn area of Baltimore City's Southern District. This is a large area, and to police it safely and effectively you need about seven to eight officers per shift," said Mike Mancuso, president of the Baltimore City Fraternal Order of Police Lodge 3.

Mancuso said about 2,800 officers are needed to effectively police the city, but staffing is down to about 2,100.

Worley said Sunday afternoon that the department hadn't been aware in advance that the event was happening, and he said organizers hadn't received a permit. He said that the department would examine its response.

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Ramer reported from Concord, New Hampshire. Kimberlee Kruesi in Nashville, Tennessee, contributed to this report.