The envelope was opened Thursday afternoon at the headquarters of the Council on American-Islamic Relations near the U.S. Capitol. The group's spokesman, Ibrahim Hooper, says initial testing showed the substance was not harmful. Hooper says the envelope also contained a "hate message."
FBI spokesman Andrew Ames says the bureau has taken possession of the letter, which is standard in such cases.
Hooper says the group has received an increase in hate mail and death threats in the wake of recent attacks by Islamic extremists. The group has condemned those attacks.
The building was temporarily evacuated after the letter was opened.