(This story was updated to add new information.)

Authorities in Alabama sifted through evidence and dozens of tips Monday as they worked to identify "multiple suspects" who opened fire in Birmingham's busy entertainment district, killing four people, injuring 17 and leaving city residents shaken.

Around 11 p.m. Saturday, the gunmen “fired upon a large group of people” who were outside in the Five Points South district, a bustling area of shops, bars and restaurants a few blocks from the University of Alabama at Birmingham campus, according to the Birmingham Police Department.

Police said the shooters were targeting at least one person in what may have been a murder-for-hire and that several of the victims were innocent bystanders. Detectives on Monday were still working to identify the intended target of the shooting as well as the identities of the gunmen.

The Birmingham Police Department identified those killed as Anitra Holloman, 21, Tahj Booker, 27, Carlos McCain, 27 and Roderick Lynn Patterson Jr., 26.

A total of 17 people were wounded, four of whom suffered life-threatening injuries, department spokesperson Truman Fitzgerald said in an email Monday morning. At a news conference, Birmingham Police Chief Scott Thurmond said five victims remain in the hospital and 12 have been discharged.

Shooters targeted at least 1 person in possible murder-for-hire

Thurmond told reporters the shooters were targeting at least one person – who is among the victims – and that others were caught in the barrage of bullets. He also suggested that the shooting may have involved a possible murder-for-hire, citing the criminal histories of several of the slain victims.

"Some of the individuals that were killed have extensive criminal histories and because of that there's oftentimes motivation from others," he said Monday. "And there's people who are willing to pay to have them killed and so that's part of it."

Thurmond said investigators are combing through a "significant number" of tips they have received through Crime Stoppers, a tip line where people can provide information to detectives anonymously.

Investigators are also working with federal partners to "connect the dots" between separate pieces of evidence, including information gathered from witness interviews and surveillance video of the shooting, which saw at least 100 shots fired, Thurmond said.

One victim was celebrating a recent birthday, mayor says

Birmingham Mayor Randall Woodfin said at a news conference Monday that he spoke with victims and their families, including one person who was celebrating a birthday and was waiting outside a club when gunfire rang out.

"His mom mentioned to me that they usually go out of town for his birthday, but he decided to celebrate his birthday here," Woodfin said.

A mother of one of the victims asked for counseling resources, saying the incident has had a tremendous emotional impact on her daughter, the mayor said.

"This type of incident, this mass shooting, has a heavy toll on community as a whole ... nothing more harmful than the emotional and physical pain of these actual victims," he said, adding, "If there ever was a time for us to come together as a community, this is the moment."

Shooters suspected of using modified weapons

Both the police department and Woodfin said the shooters are believed to have modified their weapons using "Glock switches," devices which can convert semiautomatic handguns into automatic weapons.

"This is not the first occasion, unfortunately, in 2024 where we’ve seen the style of weapons, the number of bullets on the scene ... for automatic weapons being used in our streets," Woodfin said at a news conference over the weekend.

Alabama state legislators have pushed to codify a federal ban of the devices under state law, but that effort fell short in the last state legislative session.

Federal agencies assist in investigation; reward up to $100,000

The Bureau of Alcohol, Tobacco, Firearms and Explosives and the Federal Bureau of Investigation are assisting local police in the investigation.

The FBI on Monday announced a reward of $50,000 for information leading to an arrest and conviction.

Crime Stoppers also offered $50,000 for information leading to the solving of the case – the largest amount ever offered by Crime Stoppers in the state, according to J. Frank Barefield Jr., chairman of Crime Stoppers of Metro Alabama.

"I want to make myself clear on what the priority is: It is to hunt down, capture, arrest and convict the people who are responsible for this mass shooting," Woodfin, the city's mayor, said, calling the shooters "cowards" for firing at a crowd gathered outside a club.

US records over 400 mass shootings this year, nonprofit says

The shooting in Birmingham was the 404th mass shooting in the U.S. this year, according to the Gun Violence Archive, a nonprofit that defines mass shootings as incidents involving four or more victims.

The incident is the latest shooting to rock the city of Birmingham this year. In July, a shooting at a nightclub left four people dead. The same month, three people, including a 5-year-old child, were killed in a shooting.

At a news conference on Sunday, Woodfin urged lawmakers to ban assault weapons, saying, “Elected officials locally, statewide and nationally have a duty to solve this American crisis."

Contributing: John Bacon, USA TODAY; Marty Roney and Victor Hagan, Montgomery Advertiser