The firebombing of the clinic in Thousand Oaks, Calif., on Wednesday came 28 days after an almost identical arson at a Planned Parenthood clinic in Pullman, Wash., home of Washington State University.

In both cases, authorities say, rocks were used to smash out windows before a petroleum-based accelerant was thrown inside before ignition. Arson attacks also occurred on July 19 at a clinic in Aurora, Ill., and on Aug. 1 involving a vehicle parked at a clinic under construction in New Orleans.

No group has claimed responsibility for the firebombings, while investigators cull the Internet for clues.

At the Thousand Oaks clinic, authorities say a sprinkler system inside the clinic quickly doused the fire, limiting the damage. But at the Pullman clinic, where there were no sprinklers, the fire heavily gutted the interior of the relatively new building, rendering it uninhabitable, causing thousands of dollars in damage. It is being rebuilt, while temporary quarters have opened at another location in Pullman.

“In a nutshell, the M.O. is very, very similar,” Pullman Police Commander Chris Tennant told Hatewatch today in comparing the two most-recent clinic arson attacks.

Federal and local investigators have developed several leads in the Sept. 4 Pullman clinic arson, but no suspects have been arrested, Tennant said. Surveillance video from inside the clinic was recovered from a burned server, but it only shows flames and not the suspect responsible, the police official said.

Investigators checked video surveillance at neighboring business, but have been unable to come up with suspect images.

The arson fire in Thousand Oaks broke out about 11:30 p.m. Wednesday when no one was inside the clinic, the Los Angeles Times reported. Deputies and firefighters were quickly alerted by the building’s fire alarm.

The latest attack came the same week that Cecile Richards, president of Planned Parenthood, told a congressional hearing that the organization did not break the law.

“Planned Parenthood has been in the news recently because of deceptively edited videos released by a group that is dedicated to making abortion illegal in this country,” Richards told a House panel.

Sen. Dianne Feinstein, D-Calif., issued a statement saying the “toxic rhetoric directed at Planned Parenthood has dangerous consequences.”

“It sends a signal that using violence to close clinics and intimidate healthcare professionals and women is 'OK.' It is not,” Feinstein said.

Joy Peltier, vice president of development for Planned Parenthood of Greater Washington and North Idaho, told The Stranger, a Seattle-based alternative newspaper, that the Pullman clinic firebombing was an “act of violence.”

“Extremists who protest us regularly and who rallied outside of our health centers on August 22 aren’t just against safe and legal abortion—they’re against condoms, birth control, medically-accurate sex education and a woman’s right to control her own body,” Peltier told the newspaper.

David S. Cohen, a law professor at Drexel University and an expert in anti-abortion terrorism, told Hatewatch today that since the release of the videos, people as a whole have increased their support for Planned Parenthood.

But as “Republicans have gone on the attack …there is a small contingent of anti-abortion terrorists who are seizing on this moment to further target clinics and providers.

"It’s scary to contemplate what might happen next,” Cohen, who has written a recently released book on the topic, told Hatewatch.

Meanwhile, Tanya Riordan, a vice president with Planned Parenthood of Greater Washington and North Idaho, told Hatewatch the videos don’t expose any wrongdoing on the part of the women’s health provider, but “they are simply inciting extremists and acts of violence.”

“These attacks are meant to intimidate and threaten Planned Parenthood, our patients, and the communities we serve,” Riordan said. “This is all part of a decades-long attack by extremists making false accusations and terrorizing women and their doctors, all in order to ban abortion and cut women off from care at Planned Parenthood.”