An envelope ignited at a post office in the US capital Friday, a day after suspect packages burst into flames in state government buildings in neighboring Maryland, police said.
But Washington police chief Cathy Lanier would not confirm reports that Friday's letter was addressed to Homeland Security Secretary Janet Napolitano.
"As with yesterday, we're taking every possible precaution... Right now we don't have any other packages, but we're not taking anything for granted," she told a hastily-convened press conference.
She said the package received at the post office in northeast Washington had "been described as popping, smoking and with a brief flash of fire, and then it went out, extinguished itself."
The post office was evacuated and no one was injured, but the case has been linked to similar incidents on Thursday when packages were sent to two Maryland government offices.
The Federal Bureau Investigation has already been brought in to probe the the booby-trapped packages.
"The initial response indicates that it bears similar characteristics to the two packages yesterday in Maryland," FBI spokeswoman Lindsay Godwin told AFP.
The post office on V Street Northeast, close to Maryland's state line, is one of several that handle mail and packages sent to federal government agencies in Washington.
On Thursday two book-sized parcels were delivered to official buildings in the Maryland state capital Annapolis and a transportation building in Hanover.
The packages burst into flames, spewing out sulfurous smoke, and slightly singeing the fingers of an employee at the Annapolis building.
Local television WJZ said a message referring to recent electronic road signs erected in Maryland that urge drivers to "report suspicious activity" had been included in one of the packages.
The message warned, "You have created a self-fulfilling prophecy," according to an image in the Washington Post daily.
Maryland congressman Dutch Ruppersberger told CNN Friday that he had initially believed that Thursday's packages were the work of a "lone wolf."
But he said he was now "concerned there is a pattern."
Maryland state agencies have stopped all mail delivery until an investigation is complete.
Ruppersberger said state investigators were working with the FBI and the Department of Homeland Security.
"It seems on initial investigation that these packages are the same. Now there is a lot of investigation that has to occur, and remember the first issue you deal with is safety of your citizens, safety of your people in the mail room," he told CNN.
The next phase, he said, would be the intelligence agencies working together "to see whether or not this happens to be a conspiracy or a terrorist-type operation such as Al-Qaeda."
Maryland is one of the neighboring states to Washington, DC, and home to many daily commuters to the capital city.
US post office workers were shaken in 2001 when anthrax was mailed to prominent journalists and politicians around the country, killing five people and sickening 17 others just days after the September 11, 2001 attacks.
The "Amerithrax" investigation, the largest probe into a bio-weapons attack in US history, concluded last year that the army researcher Bruce Ivins acted alone in planning and executing the attacks.
Ivins, a bio-defense researcher at the US Army's Medical Research Institute of Infectious Diseases, committed suicide in July 2008 as FBI agents were about to bring charges against him.
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