The FBI is investigating a string of suspicious packages sent to military and intelligence sites across the greater Washington, D.C. area Monday. The packages, some of which contained traces of explosive material, were delivered by mail, but were later rendered safe. The first package was discovered at National Defense University, at Fort McNair in Washington, D.C. The parcel tested positive for black powder and an X-ray appeared to show a fuse and GPS, which put authorities on high alert Monday.

The FBI has taken custody of multiple suspicious packages sent to military locations in the Washington, DC, area, a law enforcement official says https://t.co/WjTCZ98EgB pic.twitter.com/d4W4PpCTTH

— CNN (@CNN) March 26, 2018

Law enforcement officials told NBC News “similar packages turned up at other military and intelligence locations—six in all, including the CIA’s mail-sorting facility, a White House mail-sorting facility in suburban Washington, a U.S. Navy facility in Dahlgren, Virginia, and two facilities at Fort Belvoir, Virginia—the National Geospatial-Intelligence Agency and another defense university.” CNN reports that “scanning machines at the facilities detected the suspicious materials upon receipt” and the packages are being examined at the FBI lab in Quantico, Virginia.”

Officials say they do not believe the packages are linked to the recent deadly bombings in Austin, Texas.