Federal authorities are investigating after ricin, a deadly toxin, was discovered in mail intended for the White House.

The Secret Service intercepted the envelope which was positively identified with mail meant for the White House, law enforcement sources tell NPR. The ricin never made it to White House grounds, but authorities are still searching for other undiscovered dangerous packages that may be linked to the ricin.

Law enforcement sources also said investigators are narrowing down suspects, but have made no arrests.

Both the White House and Secret Service are declining to comment.

Ricin, a toxin derived from castor seeds, has been previously sent to public officials and politicians, including to President Obama in 2013. A man from Tupelo, Mississippi was sentenced to 25 years in prison in that case.

In 2018, the FBI arrested a Utah man they believed sent castor seeds in two envelopes addressed to the Pentagon.

NPR’s Carrie Johnson contributed to this report.