The suspect believed to be behind mysterious letters sent to the White House and the Pentagon this week is reportedly in custody.

Letters that tested positive for ricin, a potentially dangerous substance, were sent to President Donald Trump, Secretary of Defense James Mattis, and Chief of Naval Operations Admiral John Richardson.

The suspect is reportedly a former Navy sailor. Authorities are in the process of clearing the individual's residence in Logan, Utah.

At least one of the envelopes, according to a Pentagon statement emailed to Business Insider, contained castor seeds, from which ricin is derived.

The suspect behind several suspicious letters that were sent to the White House and the Pentagon this week has reportedly been taken into custody.

Authorities took the suspect, previously identified as a former Navy sailor, into custody Wednesday, CNN first reported, adding that a crew has started clearing the suspect's residence.

The envelopes, which were intercepted by the Secret Service and the Pentagon's mail room staff, reportedly tested positive for ricin, a potentially deadly substance, especially in a pure, powdered form. The letters sent to the Department of Defense were addressed to Secretary of Defense Jim Mattis and Chief of Naval Operations Admiral John Richardson. The letter sent to the White House was addressed to President Donald Trump.

The suspect was identified by a return address on one of the letters sent to the Pentagon, Fox News reported Wednesday morning. ABC News, citing federal prosecutors, identified the suspect as William Clyde Allen III. He served in the US Navy from 1998 to 2002 as a Damage Control Fireman Apprentice.

While the FBI has been spearheading the investigation, the Pentagon has been providing regular updates to reporters.

"On Monday, the Pentagon Force Protection Agency detected a suspicious substance during mail screening at the Pentagon's remote screening facility," DoD spokesman Col. Rob Manning told Business Insider in an emailed statement, further explaining that "all USPS mail received at the Pentagon mail screening facility yesterday is currently under quarantine and poses no threat to Pentagon personnel."

Pentagon spokeswoman Dana White provided additional information Wednesday, revealing that at least one of the letters sent to the DoD contained castor seeds, from which ricin is derived.