(CNN) -- Police are asking for the public's help in finding whoever killed three people and wounded nine others at a crowded Southern California house party.

The melee broke out Tuesday night about two miles east of downtown Long Beach. A shooter or shooters fired on guests at the house party from an alley behind the home, Long Beach Police Chief Robert Luna said.

About 25 to 30 people were at the party, including some who were in costume, police said.

"Today is a tragic and horrific day in the city of Long Beach," Mayor Robert Garcia said Wednesday. "This was a senseless act of gun violence."

Of the 12 people shot, three men died, seven women were wounded and two men were wounded. All were between the ages of 20 and 49.

Luna said police believe the victims work together, and that most of them were not from Long Beach.

The chief said he does not believe the shooting was gang related, but says it "may have been a targeted attack."

Police are looking for a male suspect of unknown race who had dark clothing, a dark-colored vehicle and had his face concealed. Officers aren't sure whether the man acted alone.

Luna said whoever pulled the trigger is a "coward."

"The crime scene is horrific. There is blood everywhere," the chief said.

He asked anyone with information to contact police.

CNN's Madeline Holcombe and Joe Sutton contributed to this report.

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