Federal officials intercepted an envelope addressed to the White House that contained the poison ricin, a U.S. law enforcement official told The Associated Press on Saturday. The letter appeared to have originated in Canada, according to a statement from the Royal Canadian Mounted Police, which said it was assisting the FBI The letter was intercepted at a government facility that screens mail addressed to the White House and President Donald Trump, the U.S. official said. A preliminary investigation indicated it tested positive for ricin, a poison found naturally in castor beans, the U.S. official said. The U.S. official was not authorized to discuss the ongoing investigation publicly and spoke on condition of anonymity. Federal investigators were working to determine where the enveloped originated and who mailed it. The FBI, the Secret Service and the U.S. Postal Inspection Service were leading the investigation. In a statement, the FBI said agents were working to investigate “a suspicious letter received at a U.S. government mail facility” and that there is “no known threat to public safety.”