The incident occurred at approximately 11:30 p.m. Friday at the Lorenzo Apartments in the 300 block of Adams Boulevard.
A man in his mid-20s was shot in the torso and rushed to an area hospital to undergo surgery. As of 12:45 p.m., police said the man was listed in critical condition.
Investigators said he was attending a party on the fourth floor of the apartment building. Police said there were unconfirmed reports that the party was to celebrate the victim's birthday.
The victim's family says he is not a USC student. Police are still looking for the gunman, who was only described as an African American male.