A suspicious substance dropped off Thursday to a Council on American-Islamic Relations office in Santa Clara has since been declared "benign," officials said.
The FBI, however, continues to be the lead agency investigating the delivery.
In an email Saturday, Zahra Billoo, the advocacy group's executive director, thanked local and federal authorities for their "rapid assistance" as well as others who offered words of support.
"We have been positively overwhelmed by the outpouring of concern from community members, elected officials, the media, interfaith leaders, and our fellow Americans," Billoo wrote. "We will be taking increased precautions to protect our team and ensure we are able to continue our work on the frontlines."
Officers and fire crews responded to a report of an envelope containing white powder at CAIR's Bay Area office at a two-story building at 3000 Scott Blvd. at about 1:15 p.m., police Lt. Kurt Clarke said.
Firefighters responded to the scene, evacuated the building and called for a hazardous materials team that found the envelope, Clarke said. The team conducted an investigation that continued into the night with help from Santa Clara County Fire Department, according to Clarke.
Three employees at the office were taken to a hospital as a precaution and were released later that night, said Billoo, who was not at the office on Thursday afternoon.
FBI investigators who also responded to the scene Thursday collected evidence and took possession of the letter, which is being processed at a lab, FBI spokeswoman Michele Ernst said.
Earlier on Thursday, the group's headquarters in Washington D.C. was evacuated due to a "foreign substance" found in the mail, the organization said on Twitter. Preliminary tests of the substance sent to the Washington D.C. office showed it was not dangerous and the FBI will continue to test the letter.
"Our biggest concern is the rise in Islamophobia and hate attacks on American Muslims across the country," Billoo said. "We are worried about how normal this is becoming."
But the executive director said the attacks have done nothing to soften the council's resolve although its Santa Clara office was closed Friday.
"Today, we stand even more resolutely in the face of increasing Islamophobia, prepared to protect your civil rights," Billoo noted.