WASHINGTON (Reuters) – U.S. investigators have nearly ruled out terrorism after envelopes sent to a Pentagon mail sorting facility were falsely flagged for the possible presence of the deadly poison ricin, U.S. officials said on Wednesday.

A Pentagon spokeswoman said tests so far showed that the alert was triggered by castor seeds, which ricin is derived from, as opposed to the deadly substance itself.

U.S. security and law enforcement officials separately said an active counter-terrorism investigation was not being conducted into the envelopes.

Ricin is found naturally in castor seeds but it takes a deliberate act to convert it into a biological weapon. Ricin can cause death within 36 to 72 hours from exposure to an amount as small as a pinhead. No known antidote exists.

One of the letters was addressed to U.S. Defense Secretary Jim Mattis and the Pentagon said on Tuesday it had put its mail facility under quarantine. The Pentagon provided no further comment on Wednesday, referring reporters to law enforcement agencies.

(Reporting by Mark Hosenball, Idrees Ali and Phil Stewart; Editing by Phil Berlowitz)