Police said they believe the shooting in the city of Birmingham "was not random and stemmed from an isolated incident where multiple victims were caught in the crossfire".

A preliminary investigation showed "multiple suspects fired upon a large group of people who were outside in a public area" on Saturday night, Birmingham Police Department Officer Truman Fitzgerald told a news conference.

"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," Mr Fitzgerald said.

He added: "There's no one in custody... 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."

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Mr Fitzgerald said officers arrived on the scene shortly after 11pm on Saturday local time (5am Sunday UK time) and found two men and one woman "unresponsive on the sidewalk suffering from a gunshot wound".

He added they were pronounced "dead at the scene" before a third man who suffered a gunshot wound died at a hospital.

Mr Fitzgerald said four of the 18 people who were wounded have "life-threatening injuries".

The shooting took place in the Five Points South area which contains entertainment venues, restaurants and bars and is often crowded on Saturday nights.

There were no immediate arrests, and investigators had not yet determined the intended targets, Mr Fitzgerald said.