At least four people were injured when a gunman opened fire at a crowded Halloween party on the University of Southern California (USC) campus Thursday morning.

The victims were rushed to the nearest hospitals and the details of their identity or their status were not known.

The campus’s online news outlet, Neon Tommy, said the incident took place at 23:30 local time (06:30 GMT). Police confirmed that there were casualties, according to the BBC.

According to an LA Times report, authorities have detained two persons for questioning, but it was not clear whether they were suspects.

The LA Times reported the witnesses as saying that the shooting seemed to occur “out of nowhere” in the Halloween party hosted by the students at the Tutor Campus Center on USC campus.  

"Everyone started running. I saw a guy who was shot in the leg. He was limping and screaming for his friends to help him. 'I got shot. can someone give me a ride?' His friends were helping him walk," the witness told the LA Times. "People were screaming and running away."

The USC, which initially shut down the campus following the incident, later issued a statement on its website which read:  “Threat is over. No students, faculty or staff involved. Campus open to those with USC ID. Classes resume in morning.”

Meanwhile, three women were killed and several others seriously injured after a stampede at a Halloween party in Madrid, Spain, BBC reported. The cause of the stampede was not known. "There was a human crush at the only exit they had because the others were sealed off," a partygoer, who gave her name as Sandra, told Spain's Cadena Ser radio station.

In 2008, USC sprinter and now Olympic silver medalist Bryshon Nellum was shot in the leg three times at a Halloween party just off the campus.