OTTAWA, June 15 (Xinhua) -- The Royal Canadian Mounted Police (RCMP) is investigating a death threat against U.S. ambassador to Canada along with a suspicious "white powder" substance mailed to her residence in Ottawa, CTV reported Friday.

An email sent to all embassy staff said the incident happened around 10:30 a.m. Thursday after an off-site mailroom supervisor encountered white powder while screening a letter addressed to Ambassador Kelly Craft.

Fire and RCMP personnel and a hazardous materials team were immediately dispatched, and a sprawling upscale residence in Ottawa was sealed off. The substance has been deemed not to be harmful.

Along with the powder substance, a letter was also enclosed, which threatened to kill the ambassador if she did not resign from her post. The letter also made threatening comments against U.S. President Donald Trump.

Diplomatic tensions have escalated between Canada and the United States after Trump imposed tariffs on Canadian steel and aluminum, and made personal attacks against Prime Minister Justin Trudeau.