Two offices of the Council of American-Islamic Relations, known as CAIR, were evacuated Thursday after receiving suspicious letters, the group said.
The CAIR office on Capitol Hill in Washington, D.C. was evacuated and staff were briefly quarantined after receiving a letter containing "white powder," the group said.
CAIR spokesperson Ibrahim Hooper told BuzzFeed News that the letter was discovered around 1 p.m., and that employees were placed under quarantine both inside and outside the building.
"When they opened [the letter] up, a poof of white powder came out," he said.
Local firefighters and police officers had been called to the scene.
An initial testing of the substance came out negative for toxins, a fire official told BuzzFeed News.
"We've received threats on a regular basis, been awhile since we've seen something like this," he said.
Hooper added that no major events had been scheduled at the Capitol Hill office that day.
Maha Sayed, a CAIR staff attorney, said on the advocacy group's Instagram account that "it's frightening to experience the hate manifest itself to such a real level. This will not deter us from continuing to protect the civil rights and liberties of all Americans."
Later Thursday, CAIR staff were also evacuated from their offices in Santa Clara, California, after receiving a letter with "suspicious" powder, NBC Bay Area reported.
Zahra Billoo, executive director of CAIR in the San Francisco Bay Area, posted on Instagram that no one had been injured.
"Know that this will not intimidate or silence your civil rights lawyers and advocates," she wrote. "Today, we stand even more proudly in the face of increasing Islamophobia, prepared to protect your civil rights."
Billoo later tweeted that staff were being taken to a hospital as a precaution.