(CNN) -- The FBI is investigating letters containing a white powder that were mailed to at least seven federal offices in the states of Washington and Idaho, FBI representatives said Monday.
"We are involved," the FBI's Fred Gutt said in Seattle, Washington, adding that agents were "still on the scene of all of them."
Gutt said letters had been received at the downtown federal building in Seattle, the FBI office at the Rockpointe Building in Spokane, and IRS offices in Bellevue.
FBI spokeswoman Debbie Dujanovic Bertram in Salt Lake City, Utah, said similar letters had been found in Idaho at the U.S. attorney's office and the FBI office in Coeur d'Alene, at the FBI office in Pocatello and the U.S. Attorney's office in Boise.
Bertram said field tests indicated the substances were not toxic, but samples were sent back to labs for further testing.
In Bellevue, the four-story IRS building was evacuated when an employee opened a letter and the white powder "poofed out," Bellevue Police spokeswoman Carla Iafrate said. The building will remain empty until authorities identify the powder, she said. A state laboratory will conduct those tests, she said.
In Boise, the police department issued a statement saying the powder was found in an envelope mailed to the U.S. attorney's office. The Boise Fire Department evacuated only that office in the building, and allowed the employees back in when tests determined the substance was not a threat.