At least four people were killed and dozens more injured in a shooting in Birmingham, Alabama police said early Sunday.

At least four of the injured were “critically injured,” said Truman Fitzgerald, a Birmingham police officer, at a press conference a few hours after the incident in what he considered to be one of the city's “most popular entertainment districts.”

Fitzgerald said officers arrived at the scene shortly after 11 p.m. Saturday “when they observed two adult males and one adult female lying unconscious on the sidewalk with a gunshot wound.”

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

“We believe multiple shooters fired multiple shots at a group of people,” he said. “Our investigators are working to determine whether the shooters approached the victims or drove past in a vehicle. We need the public's help to do this,” he added.

“No one is in custody,” he said. “Rest assured, we will do everything in our power to ensure that we uncover, identify and bring to justice those responsible for the attacks on our people,” he added.

Fitzgerald did not speculate on a motive, but said investigators were “told that all of the victims were outside on the sidewalk or street. So this group of shooters shot at the victims while they were outside. We were not told this happened in a store.”

He also called on businesses with surveillance cameras in the area to review their footage and provide investigators with anything relevant.

The department has asked the FBI and ATF for assistance in the investigation, he said.

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

This article was originally published on NBCNews.com.