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

The chaotic scene was 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.

Investigators are trying to find whoever shot 12 people from an alley behind a house.

"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. The victims were between the ages of 20 and 49.

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