U.S. Capitol Police and a hazardous materials unit responded Thursday afternoon to the mail facility on East Hampton Drive in Capitol Heights, Md.
The letters reportedly have threatening words written on the outside. It was not immediately clear who the letters were addressed to. The facility screens mail for the House of Representatives.
There are no explosive devices or any other hazardous materials contained in the letters, according to NBC News.
An FBI spokesperson stressed that there was "no direct threat of harm" to any lawmaker or their staff, and that the FBI responds to these types of letters on a routine basis.
The FBI said there is no link to the letters containing explosives found at mail facilities last week and no connection to the shootings in Tuscon.
Last week, three packages mailed to government officials in D.C. and Maryland "ignited." An FBI terrorism task force is investigating those incidents and have not announced any suspects.
All mail to congressional offices is opened and inspected before its delivered to the Capitol.
Stay with NBCWashington.com for more information.