He served in the Marine Corps Reserves from early 2004 to late 2012 as a combat engineer, reaching the rank of corporal, according to records released by the Defense Department. Those records also indicate service that was unremarkable.
He did serve a tour of duty in Iraq, in 2005, according to his mother, Cecilia Klein. She said he never fired his weapon there.
At an initial hearing on Friday, Zia M. Faruqui, a magistrate judge in Washington, ordered Mr. Klein held until his bail hearing on Tuesday. Mr. Klein did not respond to a request for comment, and his lawyer, Michelle M. Peterson, a federal public defender, declined to comment on his case.
One person who has known him for many years said Mr. Klein was a loner who could be socially awkward. His mother said nothing in his past would have suggested him capable of joining a violent riot.
“Normal schools, decent grades, went to college,” she recalled during an interview on Friday. “It was all perfectly suburban.”
According to the F.B.I., Mr. Klein held a top-secret security clearance that was renewed in 2019. Such clearances are not unusual for military service members, and his LinkedIn page says his was granted by the Defense Department. It is not clear what responsibilities he carried that would have earned the clearance. The person who has known him for years said he seemed to conjure an air of secrecy about his military service.
Under pressure from the White House in early 2017, Mr. Klein, whose father was from Argentina and had worked at the Inter-American Development Bank before his death, was installed in the State Department’s Office of Brazilian and Southern Cone Affairs as a special assistant. After Secretary of State Rex W. Tillerson expanded an office that handles Freedom of Information Act requests, viewed by many in government as an unrewarding outpost, Mr. Klein’s superiors transferred him there.