The shooting occurred around 11:30 p.m. Wednesday outside the Ronald Tutor Campus Center, where a long line of people were waiting to be checked into the party held by the Black Student Assembly.
A short time later, two men were apprehended by campus police and turned over to Los Angeles police, said LAPD Officer Sara Faden. Investigators were still interviewing the men at midmorning, Faden said.
Two men in the line of well over 100 people were arguing when one pulled a gun and opened fire, said USC police Capt. David Carlisle.
The most seriously injured man was hit several times and underwent surgery at a nearby hospital, Faden said. She said the three bystanders were treated for minor injuries and released.
None of those shot or arrested were USC students, the captain added.
A gun was found near the shooting scene and will be tested to see if it was the weapon used, Faden said.
Carlisle said he didn’t know how many people were inside the center ballroom, but "in order to gain admission at this event, they were checking student IDs at the door. You had to have a USC student ID or an ID from another university or college."
Students arrange the parties, and the events become well known very quickly on social media sites, so unauthorized guests may have been in line, he said.
The flier for the "Freak, Greek or Athlete" party said it would last from 9 p.m. to 2 a.m., there was room for 800 and it would include rave lighting and mega sound.
USC students were admitted free while those with a valid ID from another college had to pay. Costumed guests and Greeks got a discount.
The flier also advised: "Strict Off Duty Officers plus Campus DPS (equals) No Worries."
The student alert, sent out by email, text and phone, was lifted just before 2:30 a.m., and students were advised a normal class schedule was planned Thursday.
"This is the first violent crime we’ve had on campus this year," Carlisle said.
Michael L. Jackson, the vice president for student affairs, said university policy requires that on-campus student parties be open only to "guests with student IDs from USC or another university.
"This incident reminds us that we must look out for ourselves and be particularly vigilant about the personal safety of friends and guests at our social events," he said in a statement.
In April, two USC graduate students from China were shot to death in a car about a mile from campus. Police believe Ming Qu and Ying Wu were victims of an apparent robbery.
Two men have pleaded not guilty to murder charges in the case. Trials are pending.
In other Halloween violence in Los Angeles, three people were shot around 10 p.m. on Hollywood Boulevard near the Halloween parade and party that annually draws hundreds of thousands of people.
Despite a quick police response by officers in riot gear, on horseback and bicycle who were on duty for the Halloween revelry, the gunmen got away.
Police said a 17-year-old boy was critically wounded with gunshot wounds to the chest and a leg. A 14-year-old boy was shot in the foot, and a 25-year-old man was struck in the buttocks.