LOS ANGELES -

Two people were in custody today following an outbreak of gunfire that wounded one of two people involved in an argument and three bystanders  on the USC campus while a Halloween party was under way.

No students or USC staff members were involved or injured in the violence or shooting.

Campus police had been monitoring the party and witnessed the shooting, according to a statement issued by the university.  Officers quickly took two suspects into custody.

The four victims were rushed to hospitals after the shooting, which occurred at Tutor Campus Center around 11:35 Wednesday, according to Capt. David Carlisle of the USC Department of Public Safety.

Two males, neither one a USC student, were arguing outside an on-campus Halloween party and one of them pulled a handgun and shot the other, wounding him and three bystanders, Carlisle said.

The male targeted by the shooter was taken to a hospital in critical condition and the other three victims suffered non-life-threatening wounds, he said.

Two people were detained following the shooting and were in custody this morning, Carlisle said without laboration. It was not immediately clear if either was being investigated as the possible shooter.

USC's Department of Public Safety issued a Trojan Alert following the shooting asking students to take "shelter in place and avoid opening doors for unknown persons," and to stay away from the campus, which was declared "shut down."

The lockdown was lifted around 2:30, when students were advised over USC'c Twitter account that the emergency was over, no USC students or staff had been involved, and the campus once again was accessible to those carrying appropriate identification.

The gunfire broke out while around 100 people, including some campus police officers, stood outside a jammed Halloween party, a student at the scene told the Los Angeles Times.

"Everyone started running. I saw a guy who was shot in the leg. He was limping and screaming for his friends to help him," the student said.

One of the victims was on the ground, struggling to breathe, another student told The Times.

The USC area has been the site of firearm violence before.

Two USC graduate students from China were shot and killed several miles from the campus in a botched robbery in April, prompting officials to beef up campus-area security. Two men have been charged in the slayings.

Wednesday's shooting comes four years to the day after USC sprinter Bryshon Nellum, who won a silver medal at this year's London Olympics, was shot in both legs at a Halloween party just off campus.

Classes resumed on campus Thursday morning.