The FBI is investigating after a half-dozen suspicious packages containing explosive materials were mailed Monday to military installations around Washington, D.C., according to news reports.

CNN reported that Fort Belvoir in Virginia and Fort McNair in Washington each received a suspicious package.

The box sent to the National Defense University at Fort McNair tested positive for black powder and residue and had a fuse attached, an Army official said.

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The building was evacuated, and cleared for re-entry by 1:15 p.m., CNN reported.

The package at Fort Belvoir was rendered safe.

The FBI is reportedly working to determine whether each of the boxes contained functioning explosives, or replica devices, as well as who is behind the packages. Authorities said they do not believe the packages came from the man responsible for a series of recent bombings in Austin, Texas.

The suspected Austin bomber was identified last week as Mark Anthony Conditt. Police said the 24-year-old was responsible for a series of package explosions in recent weeks, which killed two people and injured several others.

Conditt killed himself last week as police closed in on him while he was in his vehicle.