TORONTO -- An envelope addressed to the White House that contained the poison ricin may have come from Canada, investigators say, and the RCMP has been called in to investigate.

Officials intercepted the letter at a U.S. government facility that screens mail before it reached U.S. President Donald Trump. Investigators are now working to track down its source.

“Initial information from the investigation suggests that the letter originated in Canada,” RCMP spokesman Dan Brien said in a statement.

The RCMP is now working with the FBI on the investigation. Officials have not indicated where in Canada the package may have originated.

Ricin is a toxic substance that occurs naturally in castor beans and is often found in powder, mist or pellet form.

Minister of Public Safety Bill Blair acknowledged the incident in a statement to CTV News.

“We are aware of the concerning reports of packages containing ricin directed toward US federal government sites. Canadian law enforcement is working closely with their US counterparts. As this is an active investigation we cannot comment further,” Blair’s office said in a statement Saturday.

This isn’t the first time ricin has been mailed to the White House. A woman from Texas admitted to sending letters containing the poison to Barack Obama and former New York City Mayor Michael Bloomberg in 2013. She was sentenced to 18 years in prison.

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.”

With files from The Associated Press