Police believe the shooters were carrying out a hit on one individual, who is among the deceased, and the other victims were caught in the crossfire.

“Someone was willing to pay money to have that person killed,” Thurmond said.

With no suspects in custody, the Birmingham Police Department is partnering with the FBI and other agencies to investigate, urging anyone with information to come forward.

It’s possible that gun conversion devices, sometimes referred to as “switches,” were used in the shooting, Birmingham Mayor Randall Woodfin said. These devices can be used to override the trigger mechanism on a gun, so it functions as a machine gun.

The US Attorney’s office in Birmingham announced an initiative cracking down on the illegal devices in July.

Woodfin said gun violence is at an “epidemic level” in Birmingham.

“I want to work with the state to solve this problem,” he said.

When officers arrived at the scene Saturday night, they found two men and a woman unresponsive with gunshot wounds, and all three were later pronounced dead on the scene, Birmingham Police Officer Truman Fitzgerald said.

A fourth victim, an adult male, was pronounced dead at the University of Alabama Hospital, according to hospital staff. Police previously said that 18 victims were injured.

The gunfire extends the list of at least 403 mass shootings in the United States so far this year, according to the Gun Violence Archive. The archive, like CNN, defines a mass shooting as one in which at least four people are shot, excluding the shooter.

Birmingham has seen multiple mass shootings this year.

A shooting at a nightclub in the city in July left four people dead and 10 others injured, according to CNN affiliate WVRC, and four men were shot and killed outside a public library in February, CNN affiliate WVTM reported.

Five Points South is a busy area known for its restaurants, night clubs, pubs and live music venues near the University of Alabama at Birmingham campus and downtown.

The victims in Alabama were all out in the open on the sidewalk or on the streets, Fitzgerald said.

More gunshot victims were identified in the area, and other victims arrived at local hospitals, Fitzgerald said. The University of Alabama at Birmingham Hospital received 11 of the victims, including one of the people who died, Hannah Echols, a hospital spokesperson told CNN. The conditions of the victims have not been released.

At least four victims have life-threatening injuries, Fitzgerald 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 said.

Birmingham police have asked that businesses in the area provide any surveillance footage they might have.

Fitzgerald said mass shootings “have more to do with culture than they do criminality,” adding that “we’re seeing far too many arguments being settled by bullets.”