TEMPE, Ariz. (AP) -- An altercation outside a nightclub where a rap artist was scheduled to appear erupted in shooting, leaving 13 people wounded, including two in serious condition, police said Saturday.

Police were looking for two young men as possible suspects. Tempe police Sgt. Steve Carbajal said they fled after the shooting, which occurred shortly before midnight Friday in the parking lot of The Clubhouse Music Venue.

At least two of the injured were hospitalized in serious condition, while the others were treated for wounds that appeared less severe, Carbajal said. Some of the injured drove themselves to various hospitals so police aren't sure if there were other victims besides the 13.

Carbajal described the shooting as the result of "some sort of altercation" at the club, where a rap music concert was going on.

Witness Paul Butler was among the crowd waiting to get into the club late Friday to hear Los Angeles hip-hop artist rapper Nipsey Hussle perform and told KTVK-TV that he heard a series of pops as the gunfire erupted.

"And you see everybody running, couple more shots, everybody running," Butler told the station. "It was pure chaos."

Police didn't have immediate details on the disturbance, or comment on what type of weapon was used or whether they believed both of the men were suspected of firing weapons.

Police had only a vague description of two suspects wearing dark clothing who ran away from the club on East Broadway Road, located about 7 miles east of downtown Phoenix.

Hussle never took the stage, but tweeted after the shooting: "Az....y'all gotta be cool man. This ... ain't rite."

A phone call and email to his publicist weren't immediately returned.

Four hours after the shooting, the parking lot was virtually empty with only police activity apparent. All entrances to the club's parking lot were cordoned off with tape and squad cars were posted around the area.

The website for the business - located in a strip mall within two miles of Arizona State University's main campus in Tempe - says it's been in operation for six years hosting music shows nightly for all age groups.

Calls to the club went unanswered early Saturday.

Watch Nipsey Hussle's Video for 'Hussle in the House'

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