The FBI, Baltimore police and fire all responded to reports of a suspicious letter at the NAACP headquarters in Baltimore. (Algerina Perna/Baltimore Sun video)

Workers were ordered out of the Baltimore building that hosts the national headquarters of the NAACP , after authorities received reports of a "suspicious letter," but the letter was later determined to be harmless.

The FBI confirmed that agents had joined city fire and police officials in the 4800 block of Mt. Hope Dr. after receiving a report of a suspicious letter. "It's not uncommon for us to respond to something like that,"  said FBI spokesman Richard Wolf. "

At about 1:45 p.m., Wolf said authorities had determined that the letter was "a non-hazard, non-suspicious letter." He declined to say what triggered concerns about the letter, citing concerns about copycats.

Baltimore Fire spokesman Roman Clark said that hazardous materials teams had been sent to the location. 

Maj. Johnny Delgado, the commander of the Northwestern police district, said that around 11:30 a.m., people inside the bulding noticed a suspicious letter and the building was evacuated. He said people who had come into contact with the letter have been "isolated."