The Council on American-Islamic Relations said police evacuated its Capitol Hill office after a "foreign substance" was received in the mail.
Later Thursday afternoon, employees were given the all-clear to return.
The substance was a white powder that spilled out of a letter when opened by an employee, and several people inside and outside the office were being quarantined, spokesman Ibrahim Hooper said. The letter was described as hate mail.
A spokesman for D.C. Fire said Thursday the site had been cleared and that an initial field test on the substance came back negative, but added there will be further testing.
Firefighters set up a decontamination area in front of one of their trucks, complete with bleach and dish soap on the ground nearby. Firemen in full bunker gear and gas masks entered the building, and crews could be observed hosing off workers in hazmat suits.
Washington police were on the scene, and traffic was blocked off as officials investigated.
CAIR released a statement
on social media alongside pictures of authorities responding to the scene.
"We receive hate messages daily because of our advocacy on behalf of the American Muslim community," staff attorney Maha Sayed said. "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."
Communications coordinator Nabeelah Naeem told CNN that police were called after employees opened the mail and saw the substance inside. She said such suspicious items are sent to their offices "periodically" while hate mail is received "daily."
The evacuation comes amid rising tension in the U.S. over terror threats. Since the Paris terrorist attacks, there has been a renewed focus on fighting terrorism and a political debate over Islamic ideology.
GOP front-runner Donald Trump this week called for a ban on all foreign Muslims entering the U.S., which was widely repudiated by both Democrats and Republicans.
"Since the Paris attack, there has been a spike in hate crimes in the Muslim community in America," Naeem told CNN.