Residents in Hemet have long known that there is a skinhead element in their town.

Hemet City Truck burned.

They see them walking down the street, groups of tattooed, shaved-headed young men in combat boots and fatigues.

But now, authorities are investigating whether white supremacists are behind a series of attacks on police department facilities in the Riverside County suburb.

More than 150 law enforcement officials raided various sites in the area Tuesday, arresting more than 35 people. Sources have told The Times investigators believe the attacks - including booby trapping police offices - were the work of a white supremacist gang with roots in the area.

Law enforcement officials have been cracking down on these types of gangs in recent years.

Experts who study hate groups said Hemet and surrounding communities are particularly fertile ground for white supremacists, and estimated there are at least a dozen such groups operating in the Inland Empire area. Brian Levin, director of the Center for the Study of Hate and Extremism at Cal State San Bernardino, said there have been several hate incidents as well as leaflet distributions by white supremacist groups in Riverside County in the last two years, including several believed to be tied to the election of President Obama.

"There is significant concentration of hate groups in the Inland Empire unlike anywhere else in the nation from the National Socialist Movement to the Hammerskins to COORS family skins," he said.

Ten days after the 2008 presidential election, a 19-year Latino man was beaten by a group of white men in Hemet. Seven members and associates of the COORS ("Comrades of our Racist Struggle") Family skinheads were arrested by Hemet police and four were prosecuted.

A year ago, the San Bernardino County Sheriff's Department arrested members of the Inland Empire Skinheads gang in connection with several home-invasion robberies and an attempted murder case.

Hemet authorities declined to say what groups were under investigation. The sources, who spoke on the condition of anonymity because it was an ongoing investigation, also refused to cite specific groups.

The attacks began last year as booby traps were set at the headquarters of the Hemet-San Jacinto Valley Gang Task Force, officials said. In December, a utility line was redirected to fill the offices with gas. Officials said a spark could have triggered a devastating explosion.

In February, a "zip gun" was hidden by the gate to the task force office and rigged to fire. When a gang officer opened the gate, the weapon went off and the bullet narrowly missed him, authorities said. In early March, police said, a "dangerous" device was found near the unmarked car of a task force member. That was followed by an arson attack on four city code enforcement trucks March 23.

There have been many theories over the months about who was responsible for the attacks.

Last month, authorities arrested 33 alleged members of the Vagos motorcycle gang. After the operation, Riverside County Dist. Atty. Rod Pacheco said in an interview that the Vagos were "an extreme threat to law enforcement." But authorities have never formally said that gang was involved.

Tracie Long, a 38-year-old Hemet sales clerk, said she's seen skinheads in the city but said she's had no dealings with them. She said dozens recently congregated on a major street corner to decry pending immigration legislation.

Long said she was skeptical that Tuesday's arrests would solve the mystery of the attacks. "Until they can prove it no one knows for sure" she said.