The letter was identified and confiscated at a separate center before reaching the White House
U.S. officials say a poisoned letter sent to President Donald Trump's White House was returned before he could reach the building.
Officials say the letter was found last week at a center where all White House letters and parcels are examined.
They added that a toxic substance called resin was found in the envelope of the letter. President Trump's administration has not yet commented.
The Federal Bureau of Investigation (FBI) and the National Security Agency are now investigating who sent the letter.
They also want to know if other similar letters were sent through the US Postal System.
The FBI told CNN on Saturday that there was no immediate threat to the public.
An official told the New York Times that the letter was believed to have been sent from Canada, and that FBI tests had shown that the envelope contained resin.
Canadian police have also agreed to assist the FBI in investigating the incident.
