Police received reports of shots fired shortly after 11 p.m. Saturday at a home in the 2100 block of Spencer Avenue.
Responding officers found three victims in their early 20s with gunshot wounds. Also, a fourth victim had been punched by the suspect.
Police said the suspect was being escorted out of the party for fighting when he pulled out a gun and opened fire.
Two of the shooting victims were rushed to the hospital. One victim, identified as Tomas Mejia, was listed in critical condition. Authorities said Mejia is in his teens. Another victim, identified as 20-year-old Anthony Richmond of Rancho Cucamonga, was listed in stable condition.
The third gunshot victim, identified as 22-year-old Joshua Sotello, and the victim who was punched, 21-year-old Neyra Carrillo, were treated at the scene and released.
Investigators said there were about 100 people at the party, ranging in age from the teens to the 20s.
Authorities did not have a suspect in custody and did not have a suspect description. The family that lives in the house where the party was held said they don't know the suspect and don't know how he got into the party in the first place.
If you have any information regarding this incident, you're urged to contact the LAPD We Tip hotline at (800) 78-CRIME or (800) 782-7463.
City News Service contributed to this report.