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.