The FBI says initial testing has established a link between the three fiery packages sent to officials in both the federal government and the state of Maryland.
The packages were discovered earlier this month at a mail facility in the District and two state office buildings in Maryland.
The package that appeared in D.C. was addressed to Homeland Security Secretary Janet Napolitano. The Maryland packages were addressed to Gov. Martin O Malley and the state's transportation secretary.
The FBI now says initial forensic tests have linked all three packages. Investigators also say the fiery letters were manufactured in the same manner.
The FBI says fingerprint and fiber analysis are expected to be completed soon. Investigators are chasing down more than 100 leads in the case and are trying to determine what motivated the sender.