Federal agents have reportedly launched an investigation after a package containing a lethal poison was sent to Donald Trump.

Law enforcement officials told CNN the package was intercepted earlier this week at a mail-sorting facility outside the White House. Sources say two subsequent tests were performed, which ultimately confirmed that the package contained ricin—a highly toxic substance found in castor beans.

According to the New York Times, the FBI is now trying to find out who sent the package addressed to the president, and determine whether other ricin-containing packages were sent through the mail. At this time, officials reportedly believe it came from Canada.

The White House has not publicly responded to the report.

In 2018, Utah resident and Navy veteran William Clyde Allen III was arrested for allegedly sending ricin-laced letters to the White House and Pentagon; however, they were all intercepted before they were delivered. In 2013, The Walking Dead actress Shannon Richardson pleaded guilty to sending ricin-containing letters to high-ranking politicians, including former president Barack Obama and former New York City Mayor Michael Bloomberg.

"What's in this letter is nothing compared to what I've got in store for you mr president," the letter addressed to Obama reportedly read. "You will have to kill me and my family before you get my guns. Anyone wants to come to my house will get shot in the face."

Richardson, who initially accused her husband of sending the letters, was sentenced to 18 years in prison.