A pre-dawn blaze that heavily damaged a Planned Parenthood clinic was arson, fire investigators in Washington state determined Friday.

Pullman Fire Department investigator Chris Wehrung also said a task force involving local agencies was investigating with the help of federal agencies. He declined to release other details.

The fire follows a recent wave of protests at Planned Parenthood clinics across the country. On Aug. 22, some 500 protesters gathered outside the Pullman clinic, waving signs and calling for Congress to defund the organization.

Many protesters referenced a series of videos released by anti-abortion activists in California that appeared to show Planned Parenthood executives saying the organization profits from the sale of aborted fetal tissue.

Planned Parenthood denied the allegations and contended the videos were selectively edited.

No one was injured in Friday's fire, which started around 3:30 a.m. in the college town of Pullman.

"This is an appalling act of violence towards Planned Parenthood, but unfortunately a predictable ripple effect from the false and incendiary attacks that fuel violence from extremists," Karl Eastlund, CEO of Planned Parenthood of Greater Washington and North Idaho, said in a statement.

The fire may keep the clinic closed for a month or more, Planned Parenthood spokeswoman Tanya Riordan told The Spokesman-Review (http://bit.ly/1PQDFNr ). The organization will be looking for a temporary office in Pullman, she added.

The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives are involved in the investigation because of the history of anti-abortion activists targeting health care providers offering abortion services, FBI Special Agent Frank Harrill said.

In 1996, a Planned Parenthood clinic in Spokane Valley was bombed. Four men were convicted in the blast. They were members of a white supremacist, anti-abortion group.

Riordan said Planned Parenthood will be looking for a temporary office in Pullman so it can resume services as quickly as possible.

___

Information from: The Spokesman-Review, http://www.spokesman.com