A suspicious letter sent to the White House has tentatively tested positive for cyanide, a deadly chemical that makes the body unable to use oxygen, the Associated Press reported.

The envelope was was received on Monday at an off-site facility that screens mail addressed to the executive mansion, and initial biological testing had come back negative, Secret Service spokesman Robert Hoback told the news service. Subsequent analysis, however, returned a "presumptive positive" for cyanide on Tuesday.

"The sample was transported to another facility to confirm the results," Brian Leary, another spokesman for the agency, told CNN in a statement. Given that the investigation into the mail piece is ongoing, the Secret Service would have no additional comments, he added.

An unidentified law-enforcement official told the Atlanta-based news channel that the individual who had opened the letter at the mail-sorting facility had not been injured and that there were no exposure concerns.

The incident had first been reported by the Intercept, which published an internal law-enforcement alert detailing that the suspicious envelope contained "an unknown milky substance, in a container wrapped in a plastic bag."

The Secret Service, which reports to the Department of Homeland Security, meanwhile, is looking into additional ways of protecting President Barack Obama and other high-ranking U.S. and foreign dignitaries.

The agency last week made news when it tested drones around the White House; now, Secret Service Director Joseph Clancy is asking for an $8 million replica of the executive mansion to better train his agents, CNN reported.

Clancy on Tuesday outlined his request during a hearing of the House Appropriations Committee on his agency's budget, the news channel noted.

"Right now, we train on a parking lot, basically," he said. "We put up a makeshift fence and walk off the distance between the fence at the White House and the actual house itself. We don't have the bushes, we don't have the fountains, we don't get a realistic look at the White House."

The Secret Service chief would like for the replica to be built in Beltsville, Maryland, about 20 miles from the real mansion in the nation's capital, NBC News detailed.