FBI Spokeswoman Lindsay Godwin said initial information indicated the parcel that ignited in northeast Washington about 2:45 p.m. was similar to the two packages opened in Maryland on Thursday. The Washington postal facility was evacuated and no injuries were reported.
The Maryland packages contained a note railing against highway signs urging motorists to report suspicious activity, investigators revealed.
The message read: "Report suspicious activity! Total Bull----! You have created a self fulfilling prophecy."
The packages, addressed to Gov. Martin O'Malley and to Transportation Secretary Beverley Swaim-Staley, have been taken to the FBI lab in Quantico, Va., for forensic analysis.
They were opened within a 15-minute period Thursday at state government buildings 20 miles apart.
The workers who opened the packages singed their fingers, but there were no significant injuries.
Soon after, mailrooms across Maryland were cleared and two other suspicious packages uncovered, though they turned out to be a toner cartridge and laptop batteries.
Explosive material wasn't found in either package that ignited and authorities aren't sure if any other dangerous packages are out there, but mailroom employees were back at work Friday. They had pictures of the packages and were advised to be vigilant about anything suspicious.
Postal Service spokeswoman Joanne Veto said the suspicious D.C. package was discovered after another package was thrown on top of it. She said the labels, postmark and stamps were similar to the Maryland parcels.
Police have not yet identified any suspects.
