US authorities have intercepted an envelope addressed to US President Donald Trump that contained a substance identified as the poison ricin, US media reported Saturday. The letter was discovered earlier this week and did not reach the White House, according to the New York Times and CNN. The Times said it was believed that the letter was mailed from Canada. Mail addressed to the White House is first inspected and sorted in depots just outside Washington. Ricin, which is produced by processing castor beans, is lethal even in minute doses if swallowed, inhaled or injected, causing organ failure. It has no known antidote.