The letter, which included white powder, was turned over to the FBI for analysis after being discovered at the Jeffrey Building. At least three governors across the country have received similar letters in the past week, Maryland officials said.
The letters, which originate out of Tennessee, could be associated with anti-war protesters, according to officials.
Similar packages have been sent to National Guard bureaus and reserve facilities in 36 states, federal authorities revealed Wednesday.
An internal Dec. 16 report from the Department of Homeland Security said the 51 packages included anti-war compact discs. Initial reports from the Guard that one of the packages contained a powdery substance turned out to be incorrect, officials said.
All the packages were postmarked from Tennessee, according to Homeland Security, and they started arriving at the Guard facilities on Dec. 12.
The FBI is investigating these and similar incidents.
Fifteen U.S. embassies in Europe have also received letters containing a suspicious white substance, and tests have shown 14 of them to be harmless, State Department spokesman Robert A. Wood said Wednesday. Test results for the substance in one of the letters has not yet been received, he said.
More than 40 governors' offices nationwide have also gotten the letters, which contain an unspecified note, FBI spokesman Rich Kolko said Tuesday.
The FBI said that all of those were postmarked from Texas; the letters began showing up in states last week. They all appear to be from the same source and have tested negative for any dangerous toxin or other threat, authorities said.