Originally Published: 06 JUN 22 01:18 ET

Updated: 06 JUN 22 11:10 ET

By Kelly McCleary and Emma Tucker, CNN

(CNN) -- A night of revelry on Philadelphia's South Street turned chaotic Saturday as gunmen opened fire into a crowd, leaving three people dead and 11 wounded in one of at least 10 weekend mass shootings across the United States.

Hundreds were "just enjoying South Street, as they do every single weekend, when this shooting broke out," Philadelphia Police Inspector D.F. Pace said. Now, a search for the shooters is underway, and police are urging witnesses to come forward, offering a $20,000 reward for information leading to an arrest.

Gun violence also marred the weekend in Chattanooga, Tennessee -- where a shooting and its aftermath left three dead and 14 injured -- and in attacks in Summerton, South Carolina, and Phoenix that each left one dead and at least seven wounded. Those incidents followed other mass shootings that have stunned the nation, including at a supermarket in New York; an elementary school in Texas; and a hospital in Oklahoma.

At least 246 mass shootings have been recorded in America this year, according to Gun Violence Archive, which like CNN defines a mass shooting as one in which four or more people are shot, not including the shooter. In that deadly wake, the US Senate gears up again this week to debate how to address the problem.

Here's what we know about the shooting that erupted in Philadelphia's popular entertainment district:

How the shooting unfolded

Uniformed officers patrolling the area known for bars, restaurants and shops heard gunfire just after 11:30 p.m. and saw "several active shooters shooting into the crowd," Pace said.

The officers "observed several civilians suffering from gunshot wounds, lying on the sidewalk and in the street," Philadelphia Police Commissioner Danielle Outlaw said Sunday at a news conference.

An officer who spotted a man shooting into the crowd drew his weapon and "fired several times in the direction of the unknown male, we believe striking him," Outlaw said. The man then dropped his gun, ran and got away, officials said.

Officials believe the shooting started after what may have been a "physical altercation," Outlaw said. "At this point we don't know whether or not the altercation that took place was between a group of individuals or if this was tied to another group or affiliation," she added.

Five guns were fired in the melee, police said, and two handguns were recovered at the scene. One of the guns police found had an extended magazine, Pace said.

State and federal authorities are helping Philadelphia police with the investigation, Outlaw said.

Who was struck by bullets

The 14 people struck by gunfire range in age from 17 to 69.

Police identified those who died as Gregory Jackson, 34; Alexis Quinn, 27; and Kristopher Minners, an educator who was celebrating his 22nd birthday with family and friends.

Minners worked with 2nd and 6th grade students at Girard College, a boarding school which educates students from families with limited financial resources, according to its website.

"He was doing an amazing job for us" and was named resident adviser of the month in March, the school said in a statement. "Kris was a vital member of our community, and his loss will be felt deeply."

It's believed one of the suspected gunmen is among the three people killed, Outlaw said. The other two victims are believed to be innocent bystanders.

One of the 11 people injured was in critical condition Sunday evening, the police department said in a news release.

How officials are responding

The shooting marked "a dark day for Philadelphia," the police commissioner said. "While many of us were out enjoying the beautiful day in the city, a horrendous and unthinkable act happened in a very popular local and tourist hangout."

There was an increased presence of uniformed officers Saturday on the streets of Philadelphia, Outlaw said, due to several scheduled events. There were even more officers deployed Sunday evening, officials said.

"People should not be afraid," Outlaw said Sunday. "What happened last night was an atrocity. But it's not something that we see all the time. And again, I don't want us to normalize this. This is not something that's normal in the city of Philadelphia, and I don't want anyone to begin to think that it is."

A curfew was implemented in part of downtown Philadelphia from 8 p.m. Sunday until 6 a.m. Monday, the mayor's office said, citing "patterns of recent violence in the area and ongoing safety concerns." The order barred all pedestrian and vehicular traffic with exceptions only for residents, local property owners and employees, licensed medical personnel, members of the media and law enforcement.

"Once again, we see lives senselessly lost and those injured in yet another horrendous, brazen and despicable act of gun violence," Philadelphia Mayor Jim Kenney said Sunday, noting it left him "not just heartbroken, but angry."

"I will continue to fight to protect our communities and urge others to advocate for stronger laws that keep guns out of the hands of violent individuals," he said.