CNN  — 

Six people were shot, and two of them suffered life-threatening injuries, following an altercation between two groups of people in downtown Chattanooga, Tennessee, on Saturday night, police said.

Patrolling officers heard gunshots around 11 p.m. in the Cherry Street area and arrived at a scene of “multiple parties exchanging gunfire and numerous people fleeing the area,” said police.

“Two individuals from one group started firing upon the other group,” Chattanooga Police Chief Celeste Murphy said in a news conference Sunday. “They believe that there wasn’t one intended target – at least in that other group – and all of the other victims that were shot were unintended.”

Investigators believe the shooting was not gang-related, Murphy said.

Multiple gunshot victims were taken to a hospital for treatment, said police Sgt. Jeremy Eames. Four are expected to survive while two have life-threatening injuries, he said. Most of those shot were teenagers or in their early 20s, he added.

“We had large groups of juveniles walking around the downtown area this date and we believe it’s from within that group that the shooting took place,” he said.

One person detained in connection to the incident has since been ruled out as a suspect and no one is in custody as of Sunday afternoon, police said at the news conference.

Chattanooga Mayor Tim Kelly, who was at the news conference, said he was “experiencing a wide range of emotions” following the shooting. “I’m heartbroken for the families and victims whose lives were upended last night by gunfire,” he said.

The mayor also asked that parents be responsible for knowing where their firearms are and store them properly out of their children’s reach.

“If you know your kid has access to a firearm, you must intervene before someone, perhaps even your own child, ends up dead,” he said.

Kelly said he was joining other mayors from across the United States calling on the Senate to pass reforms to gun safety laws.

Murphy said investigators are reviewing all videos related to the incident.

There have been 221 mass shootings so far this year, according to the Gun Violence Archive. A mass shooting is defined as an incident with at least four people shot, excluding the shooter. The shooting also comes days after an 18-year-old used a legally purchased AR-15-style rifle to kill 19 children and two teachers at an elementary school in Uvalde, Texas, again raising questions about whether such weapons should be legally available.

The location of Saturday night’s shooting is less than a mile from the Tennessee Aquarium and the city’s waterfront.

Patrick Hickey, who was driving for Uber and Lyft on Saturday night, told CNN he was parked in his usual spot waiting for another ride when he heard at least two dozen gunshots. Hickey works as a volunteer firefighter so he jumped into action.

“I helped with two victims, one of them being with a head injury.” said Hickey. “I couldn’t run in the other direction or drive away knowing there were a lot of people hurt.”

One of the victims ended up on a sidewalk near his car, so he grabbed a shirt from his vehicle and applied pressure to the wound, he said.

“(People were) screaming for help, relatives of victims were there, too,” he said. “Police were trying to get everyone back so we could help victims. It was chaos for a minute.”

Hickey added that on Sunday morning he noticed his shoes had blood on them from one of the victims.

“I’m blessed to be able to go home to my family unharmed, but it was a very scary situation.” said Hickey in a Facebook post. “Please hug your family extra close because this is becoming a common thing in the USA.”

Correction: A previous version of this story misstated Patrick Hickey's role as a first responder. He is a volunteer firefighter.

CNN’s Eric Levenson and Tina Burnside contributed to this report.