An envelope addressed to the White House contained a substance identified as ricin, a deadly poison, US media outlets reported on Saturday, citing law enforcement officials.
The package was intercepted "earlier this week", according to a CNN report citing two unnamed law enforcement officials.
CNN reported that two tests were done on the envelope to identify the lethal substance.
Citing an unnamed law enforcement official briefed on the matter, The New York Times reported that letters were sent "in recent days" to the White House and to federal agencies in the US state of Texas.
Investigators believe the package was sent from Canada and have identified a woman as a suspect, the newspaper reported.
The incident is currently under investigation, the Times said, and investigators are looking into whether more such envelopes have been mailed.
Asked about the reports, the FBI said the agency and "US Secret Service and US Postal Inspection Service partners are investigating a suspicious letter received at a US government mail facility", Reuters news agency reported.
"At this time, there is no known threat to public safety," the FBI said in its statement.
The White House and the US Secret Service declined to comment on Saturday, Reuters said.
Ricin is a poison found in castor beans, the US Centers for Disease Control and Prevention says on its website, and there is no antidote for the deadly substance.
