The US Muslim advocacy group Council on American-Islamic Relations said on Thursday its Capitol Hill office was evacuated by police after a foreign substance was received in the mail.
The office received an envelope containing a note that read, "Die a painful death, Muslims," and a white powder, according to CAIR staff attorney Maha Sayed.
"Our fear is at a pretty high level at this time, given the anti-Muslim rhetoric going on," said Sayed.
Sayed said on a posting on the group's Facebook page that the organisation receives hate messages daily.
"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," the post said.
About three employees came into contact with the powder and were quarantined in the building while police and firemen arrived on the scene to investigate, said Ibrahim Hooper, a CAIR spokesperson, adding that the rest of the office staff waited outside on the sidewalk.
CAIR's Instagram page showed the Washington office blocked off with yellow police tape and a group of emergency vehicles blocking the street.
The incident came days after Republican presidential candidate Donald Trump issued a call to prevent Muslims from entering the United States until the government could set up better screening for those seeking to enter the country.