A package addressed to US homeland security secretary Janet Napolitano has ignited at a Washington postal centre - a day after fiery packages sent to Maryland's governor and transportation secretary burned workers' fingers.
Authorities said they were expecting more packages to surface.
"Right now we don't have any other packages, but we're not taking anything for granted," DC police chief Cathy Lanier said.
Initial information indicated the parcel that ignited in north-east Washington at about 2.45pm on Friday was similar to the packages opened on Thursday in Maryland, a state adjacent to the US capital, authorities said.
The Washington mail centre was evacuated after an employee discovered a parcel that looked similar to the Maryland packages. 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."
At the DC centre, which primarily handles mail for the US government in Washington, workers are not allowed to open packages, postal inspector Frank Schissler said, so it was unclear exactly how the parcel ignited.
The package was "popping, smoking and there was a brief flash of fire", Ms Lanier said - very similar to what authorities described in Maryland. The labels, postmark and stamps were also similar to the Maryland parcels, Postal Service spokeswoman Joanne Veto said.
The DC package that ignited was addressed to Ms Napolitano, according to a Homeland Security Department official. The earlier packages, addressed to Maryland governor Martin O'Malley and transportation secretary Beverley Swaim-Staley, have been taken to the FBI lab in Quantico, Virginia, for forensic analysis. They contained no explosive material.