Witnesses are urged to come forward after shooters left 3 dead and 11 wounded in a Philadelphia crowd

(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.  

The carnage came as 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.