A manhunt has been launched for “multiple shooters” who killed four and injured 18 more when they “fired upon a large group of people,” in Birmingham, Alabama police said early Sunday.

At least four of the wounded have “life-threatening injuries,” Birmingham Police Department Officer Truman Fitzgerald told a news conference, hours after the incident in what he called one of the city’s “most popular entertainment districts.”

Fitzgerald said that officers arrived on the scene shortly after 11 p.m. Saturday “when they observed two adult males and one adult female who was unresponsive on the sidewalk suffering from a gunshot wound.”

Medics pronounced them “dead at the scene,” he said, adding that the fourth victim was pronounced dead at the nearby UAB hospital.

“We believe that multiple shooters fired multiple shots on a group of people,” he said. “Our detectives are working to confirm whether the shooters walked up on the victims or drove by in a vehicle, and that’s why we need the public’s help,” he added.

“There’s no one in custody,” he said. “Rest assured, we are going to do everything we possibly can to make sure that we uncover, identify and hunt down who was ever responsible for preying on our people,” he added.

Fitzgerald did not speculate on a motive, but said that investigators were “told that all of the victims were out in the open, on either a sidewalk or the street. So these group of shooters fired upon these victims when they were out in the open, we were not told that this happened inside of any business.”

In a later statement, police said they believed the shooting “was not random and stemmed from an isolated incident where multiple victims were caught in the crossfire.”

He also urged businesses with security cameras in the area to check their footage and provide anything relevant to investigators.

The department said it was working with the FBI and ATF on the investigation.

It is the second mass shooting in the city this year. In July, four people were killed at a nightclub.