WASHINGTON — The FBI arrested a Utah man Wednesday in an episode that federal officials initially believed was a mail attack using ricin, a deadly poison, but which they later determined involved sending castor seeds to senior government officials.

A spokeswoman for the U.S. Attorney in Utah said federal prosecutors authorized a probable cause arrest of William Clyde Allen III, 39, and plan to file charges against him in federal court on Friday.

One of two envelopes suspected of containing ricin was addressed to Defense Secretary Jim Mattis. 2013 Reuters photo

Capt. Tyson Budge, spokesman for the police department in the town of Logan, Utah, said federal agents took the man into custody at his home on Wednesday afternoon.

The arrest occurred a day after the Pentagon and Secret Service disclosed that a series of suspicious packages was detected earlier in the week that initial tests suggested contained ricin. They included two envelopes addressed to Defense Secretary Jim Mattis and Adm. John Richardson, chief of U.S. Naval Operations, and another intended for President Trump.

Dana White, chief Pentagon spokesperson, said the substance detected on the two envelopes addressed for Mattis and Richardson, which were found at a Defense Department facility in northern Virginia, contained the seeds, which are used to make ricin, but by themselves are not dangerous.

“The FBI is still investigating,” she said in a statement.

Budge said Allen has lived in Logan since at least 2016.

Ricin, typically used as a poison in mist, pellet or powder form, is created as a by-product of processing castor beans. The beans themselves can be harmful if chewed or swallowed, according to the Centers for Disease Control and Prevention.