This is reported by RIA Novosti with reference to court documents.
The detained man was a 46-year-old native of Pennsylvania.
In one of the published threats, the man called his only goal "to kill as many feds as possible before he himself perishes."
Information about what punishment threatens the detainee is not given.
Earlier, Politico reported that the FBI was investigating Trump for potential violations of the espionage law.
It is with this that the searches in his estate Mar-a-Lago are connected.
Agents found 11 sets of classified documents.
Trump demanded the return of documents seized by FBI agents.