In an affidavit, David S. Gabriel, an F.B.I. special agent, said the bureau was made aware of the activities of Mr. Hopkins after receiving reports in October 2017 of “alleged militia extremist activity” in northwestern New Mexico.

Mr. Gabriel said that the following month, two F.B.I. agents went to a trailer park in Flora Vista, N.M., where Mr. Hopkins was living at the time. With Mr. Hopkins’s consent, the agents entered the home and saw about 10 firearms in plain view, in what Mr. Hopkins referred to as his office.

Mr. Hopkins, who has also used the name Johnny Horton Jr., told the agents that the guns belonged to Fay Sanders Murphy, whom he described to agents as his common-law wife, according to the affidavit. The agents collected at least nine firearms from the home as evidence, including a 12-gauge shotgun and various handguns.

The court affidavit gave few details about the report the F.B.I. received stating that the United Constitutional Patriots “were training to assassinate George Soros, Hillary Clinton, and Barack Obama because of these individuals’ support of Antifa.” The term Antifa refers to left-wing activists who have clashed with right-wing groups in cities across the country.

The F.B.I. also did not specify why agents waited to arrest Mr. Hopkins after finding his home awash in guns in 2017. At that time, agents concluded that his possession of the weapons was illegal because he had a previous felony conviction. Separately, a probation officer in Oregon had already asked in 2007 for a warrant for Mr. Hopkins’s arrest after he failed to report to probation meetings.

Mr. Hopkins’s lawyer, Kelly O’Connell, disputed the reports about assassination plans. “My client told me that is not true,” Mr. O’Connell said.

He also questioned the timing of the arrest. “My question is, why now?” Mr. O’Connell said. He suggested that pressure from prominent Democrats in New Mexico may have prompted the F.B.I. to take action.