A package addressed to Trump containing the deadly poison ricin was intercepted this week

A letter that tested positive for the deadly poison ricin was detected at an off-site facility that screens mail addressed to the White House, law enforcement officials told NBC News Saturday. 

The letter tested positive in both a field test and a laboratory test, one official said, according to NBC News. It's unclear when the letter was intercepted. 

The Federal Bureau of Investigation said in a statement that the Secret Service, and U.S. Postal Inspection Service are investigating the origins of the letter. The letter was intercepted at the screening facility and was not sent to the White House.

Investigators believe the letter was sent from Canada, The New York Times reported. 

Other letters that also tested positive for ricin were discovered elsewhere, but only one was apparently addressed to a political figure. The others were addressed to prisons, a law enforcement official said. The official did not say how many letters were found.

Representatives of the White House and the Secret Service did not immediately return CNBC's request for comment. Representatives of the Postal Inspection Service also could not be immediately reached for comment. 

"The FBI and our U.S. Secret Service and U.S. Postal Inspection Service partners are investigating a suspicious letter received at a U.S. government mail facility," the FBI said in a statement on Twitter. "At this time, there is no known threat to public safety."