A woman has been arrested in the case of the ricin-laced package that was sent to President Donald Trump.

She was picked up as she tried to cross from Canada into the United States, CNN reported.

Authorities intercepted a package addressed to Trump earlier this week and found ricin, which is often used in rat poison. Derived from castor seeds, it has been used in terror plots and comes in powder, pellet, mist or acid form.

The poison can cause nausea, vomiting and internal bleeding of the stomach and intestines, followed by failure of the liver, spleen and kidneys, and death by collapse of the circulatory system.

The poison was discovered at a mail-sorting facility away from the White House and never got near Trump.

A law enforcement official told CNN that the woman was carrying a gun and that charges were expected against her in Washington.

The woman's identity was not released.