The RCMP is investigating a death threat sent to United States' Ambassador to Canada Kelly Knight Craft.

A source with direct knowledge of the matter told CBC News the threat, first reported by CTV News, came in the form of a letter addressed to Craft's residence but intercepted yesterday at an offsite security screening location.

Along with the threatening letter, embassy staff found a suspicious substance that was tested and found to be harmless.

The source called the incident "jarring" but said the ambassador is doing fine and pointed out that diplomats have received such threats in the past.

The Prime Minister's Office confirmed Justin Trudeau spoke about the situation with the ambassador on Friday.

​Foreign Affairs Minister Chrystia Freeland also met with Craft on Friday and called the situation "wholly unacceptable."

"She does an essential and difficult job and Canada respects her service," Freeland tweeted.

Tensions between Canada and the U.S. are running high lately, as the two countries dig their heels in on steel and aluminum tariffs and the North American Free Trade Agreement.