U.S. authorities arrested a former Marine after a February raid on the North Korean embassy in Spain, The Washington Post reported Friday, citing two people familiar with the matter.

Christopher Ahn, a former Marine and member of a group known as Free Joseon that aims to overthrow North Korean leadership, was arrested Thursday. He appeared in federal court Friday and a judge agreed to seal his case, the Post reported.

Federal authorities also raided Free Joseon leader Adrian Hong's apartment, sources told The Post.

The arrest came after a raid in which masked individuals tied up staffers, took computers and escaped to the U.S., sources told the newspaper. Free Joseon reportedly claimed responsibility for the raid in March after a Spanish judge accused Hong and two others of engaging in the embassy raid.

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An FBI spokesman referred The Post to the Justice Department for information. A Justice Department spokesperson declined The Post's request for comment. The department did not immediately respond to a request for comment from The Hill.

"We are dismayed that the US Department of Justice has decided to execute warrants against US persons that derive from criminal complaints filed by the North Korean regime," read a statement attributed to "Ambassador Lee Wolosky" on Free Joseon's website. The Post identified Wolosky as Hong's lawyer.

"The last US citizen who fell into the custody of the Kim regime returned home maimed from torture and did not survive," the statement continued, referring to college student Otto Warmbier, who died in 2017 after returning to the U.S. from North Korea in a coma.

"We have received no assurances from the US government about the safety and security of the US nationals it is now targeting," Wolosky added.