A parcel that ignited at a Washington DC postal building on Friday was similar to two sent to Maryland state offices the day before, officials say.
No-one was reported injured in the incident. The building was evacuated.
Police earlier said two parcels sent to Maryland government offices that emitted smoke when opened contained notes criticising road signs that urge citizens to report suspicious activity.
The envelope was addressed to Homeland Security Secretary Janet Napolitano.
Officials said the letter ignited at a postal sorting facility in Washington, several miles from the White House, as it was thrown by a worker into a sorting bin on Friday afternoon.
Washington DC Police Chief Cathy Lanier described the envelope as “popping, smoking, and with a brief flash of fire, and then it went out – extinguished itself”.
Without offering detail, Ms Lanier said the parcel intercepted in Washington was “similar in nature” to the two found in Maryland on Thursday.
Washington Mayor Vincent Gray said the perpetrator was “incredibly irresponsible” and described the act as “absolutely cowardly and reprehensible”.
The notes in the Maryland parcels warned authorities: “You have created a self fulfilling prophecy.”
One was opened at the Jeffrey Building in downtown Annapolis and another at the Maryland Department of Transportation building in Hanover.
Both packages, which released smoke and odours, were described as being roughly the size of books.
One was addressed to Maryland Governor Martin O’Malley and the other to state Transportation Secretary Beverley Swaim-Staley.
The remnants of the packages have been taken by the FBI for forensic analysis.
Maryland State Police, Maryland Transportation Police, the FBI and Department of Homeland Security are investigating the incidents.
The message in the packages was an apparent reference to anti-terrorism signs warning Maryland residents and highway motorists to be vigilant, and soliciting investigative tips.
“Somebody doesn’t like seeing that sign,” Mr O’Malley said on Thursday.
The contents of the Washington parcel have not been revealed.