Police have evacuated the headquarters of Council on American-Islamic Relations (CAIR) in Washington DC after the Muslim advocacy group received a "foreign substance" in mail.
Ibrahim Hooper, a spokesman of CAIR, told Al Jazeera on Thursday that the organisation did not have information on who could be behind the incident.
"We receive a lot of threats on a daily basis," said Hooper. "This is just another kind."
The organisation announced later on Thursday through its social media accounts that the preliminary field tests indicated that the substance was not dangerous.
"The letter is now in the hands of the FBI for more extensive tests," the statement said.
Maha Sayed, civil rights staff attorney of CAIR, said on the organisation's Facebook that despite being accustomed to the threats, it was 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," she said.
The organisation - the largest Muslim civil liberties and advocacy group in the US - has come under increased spotlight after the recent attack in San Bernardino that left at least 14 people killed. Its critics have accused the group of "supporting killers" despite CAIR's condemnation of the violence.
The group pleaded "to not generalise from the acts of individuals to an entire faith community".
The organisation condemned Donald Trump's call for barring Muslims from the US, calling his suggestions as "bigoted remarks".
"Donald Trump sounds more like the leader of a lynch mob than a great nation like ours," said Nihad Awad, executive director of CAIR, at a news conference in Washington DC.
CAIR says its mission is "to enhance the understanding of Islam, encourage dialogue, protect civil liberties, empower American Muslims, and build coalitions that promote justice and mutual understanding".
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Source: Al Jazeera