BIRMINGHAM, Ala. — A mass shooting killed four people and wounded 18 others in a popular nightlife area in Birmingham, Alabama, with many of the victims caught in the crossfire, police said early Sunday. No one was immediately arrested, and police asked the public for information.

The shooting happened shortly after 11 p.m. Saturday in the Five Points South entertainment district, Officer Truman Fitzgerald said in an email.

Arriving officers found two men and a woman on a sidewalk with gunshot wounds, and they were pronounced dead there. An additional male gunshot victim was pronounced dead at a hospital, Fitzgerald said.

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A preliminary investigation showed that “multiple suspects fired upon a large group of people who were outside in a public area,” Fitzgerald said.

“Detectives believe the shooting was not random and stemmed from an isolated incident where multiple victims were caught in the crossfire,” Fitzgerald said.

Injured people began showing up at hospitals, Fitzgerald said. By early Sunday, police had identified 18 other victims with injuries, some of them life-threatening.

There were no immediate arrests, and investigators had not yet determined the intended targets, Fitzgerald said. Anyone with information was asked to contact authorities, and a reward was possible, police said.

A large search for any suspects was underway, and Birmingham police reached out to federal law enforcement authorities for help in the investigation, AL.com reported.

Authorities were trying to determine whether the shooting was a drive-by or committed on foot, police told AL.com.

“Rest assured, we are going to do everything we possibly can to uncover, identify and hunt down whoever was responsible for preying on our people,” Fitzgerald said.

The Five Points South area contains entertainment venues, restaurants and bars and often is crowded on Saturday nights.