SACRAMENTO, California (AP) — Six people were killed and 12 injured early Sunday in Sacramento when shots were fired into crowds as bars and nightclubs emptied for the night in the second mass shooting in five weeks in California’s capital city.

Police were searching for at least one suspect and had recovered at least one firearm. Investigators said they were reviewing video footage posted to social media that showed what appeared to be an altercation before the gunfire erupted.

A video posted to Twitter showed people running through the street amid the sounds of rapid gunfire in the city of about 525,000 located 75 miles (120 kilometers) from San Francisco.

“This morning our city has a broken heart,” Sacramento Mayor Darrell Steinberg said. We don’t know all the facts but we know there were mass casualties in a very short amount of time.”

Officers were patrolling the area near the shooting site two blocks from the Capitol at about 2 a.m. when they heard gunfire and rushed to the scene, Police Chief Kathy Lester told reporters. They found a large crowd gathered and six people dead in the street.

Twelve people who police said were shot and wounded were taken to a hospital or hospitals. The Sacramento Fire Department said four of the seven people transported by its emergency workers were suffering from critical injuries. Authorities said some gunshot victims drove themselves to hospitals or were driven.

Authorities recovered “at least one firearm” from the scene, a police statement said, and urged witnesses or anyone with recordings of the shooting to contact police.

“We’re asking for the public’s help in helping us to identify the suspects in this,” Lester said. Asked if authorities were searching for one or more than one suspects, Lester told reporters that she did not know.

Police announced in a statement that investigators were aware of a video on social media that appeared to show an altercation before the shooting, but did not yet know whether it was relevant to their investigation.