FBI tactical and hazardous material response personnel searched the subject’s residence outside Salt Lake City.
Melodie Rydalch, a spokesperson for the US Attorney’s Office in Utah, identified the suspect in custody as William Clyde Allen, III and said federal prosecutors had authorized the arrest.
“We anticipate filing a complaint Friday in federal court in Salt Lake City,” Rydalch told CNN late Wednesday.
Allen previously served in the US Navy as an enlisted sailor. He joined the Navy in 1998 and left in 2002. He was a US Navy damage control fireman apprentice during his time in the service.
Dozens of federal and local law enforcement officials were witnessed on the scene, and an FBI public information officer in Utah confirmed an operation was underway at the Logan, Utah, residence.
According to previous reports, the suspicious envelopes were sent to Trump, Defence Secretary James Mattis and the Chief of Naval Operations, Adm. John Richardson.
An informed source told CNN that based on preliminary investigative activity, the White House and Pentagon letters were believed to be connected and the substance in question was a very crude castor bean concoction that authorities were not technically calling “ricin” until further testing.
Ricin is a highly toxic compound extracted from castor beans. It can be used in powder, pellet, mist or acid form.
If ingested, it causes nausea, vomiting and internal bleeding of the stomach and intestines, followed by failure of the liver, spleen and kidneys, and death by collapse of the circulatory system.
Dana White, chief spokesperson for the Pentagon, said in a statement on Wednesday that a preliminary analysis showed the letters contained castor seeds.
“The FBI is still investigating,” White’s statement said.