UPDATE 3-U.S. navy veteran arrested in probe of suspicious letters

A U.S. Navy veteran was taken into custody on Wednesday as a suspect in the investigation of letters sent to senior U.S. government officials initially feared to contain the poison ricin, the U.S. Attorney’s Office for Salt Lake City said.

William Clyde Allen III, 39, was arrested in Logan, Utah by the Federal Bureau of Investigation on a probable cause warrant and will be charged on Friday, said Melodie Rydalch, a spokeswoman for the U.S. attorney's office.

Allen is under investigation for letters sent to Pentagon officials and President Donald Trump, a separate law enforcement source said.

A Federal Bureau of Investigation statement in Salt Lake City said the agency was investigating "potentially hazardous chemicals" in Logan, which is about 66 miles (106 km) north of Salt Lake City.

U.S. investigators have essentially ruled out terrorism after the envelopes sent to a Pentagon mail-sorting facility were flagged on suspicion of ricin, U.S. officials said on Wednesday.

A Pentagon spokeswoman said tests showed that Tuesday's 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 of 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 U.S. Secret Service said it was investigating a "suspicious envelope" addressed to Trump that was received on Monday, though it never entered the White House.

Allen served in the U.S. Navy from October 1998 until October 2002, leaving the service as a seaman apprentice, the second-lowest rank, according to the U.S. Navy Office of Information.

(With inputs from agencies.)