SAN LUIS OBISPO, Calif. » Nine people were injured after a garage roof collapsed Saturday during an early St. Patrick’s Day party at California Polytechnic State University in San Luis Obispo that spiraled out of control and caught both city and university officials off-guard.

Several thousand students, most wearing green, poured into the neighborhood near the university Friday night and were still partying as the sun rose on Saturday.

That’s about the time when the roof with about three dozen people standing on it collapsed as screams and gasps could be heard from throughout the party.

In the most serious injury, a person’s thigh was impaled by a piece of debris early Saturday, the city said in a statement. Eight others were treated for cuts and bruises. None of the injuries are life-threatening.

Police Capt. Chris Staley says three people were arrested for misdemeanors as his department and the San Luis Obispo Fire Department brought the scene under control a few hours after the collapse.

Staley said the event, and its coming 10 days before St. Patrick’s Day, caught police by surprise.

"This is completely unprecedented," Staley told the San Luis Obispo Tribune. "We’ve never had anything like this."

The university says it is working with police, and it has opened its own investigation that could result in disciplinary measures.

School spokesman Matt Lazier said the school’s fraternities and sororities have been on social probation and forbidden from holding parties at their own houses or elsewhere since January because of reports of possible sexual assaults.

"Once we have all the facts, Lazier said, "the university will respond swiftly and accordingly."