According to the official reports, the FBI has joined the investigation into a weekend riot in Santa Cruz, Calif., that police believe was instigated by an anarchist group.

The Santa Cruz Sentinel has reported that the event was billed as a May Day party on fliers posted at the University of California at Santa Cruz and elsewhere. About 250 people showed up Saturday to attend the unauthorized party.

It was confirmed by the FBI on Monday that it had joined the investigation.

The party was a cover for violent action by masked demonstrators who threw rocks through the windows of some downtown businesses, attacked a police car that was covered with paint and scrawled graffiti on walls. Total damage was estimated at $100,000, investigators have said.

According to the police, two calls were made to 911 reporting shootings that never occurred. The calls might have been done to lure police officers away from downtown, they have said.

It was further reported that the SubRosa coffeehouse, a self-described anarchist organization in Santa Cruz, released a brief statement Tuesday denying any part in the violence. (With Inputs from Agencies)