U.S. authorities have made the first arrest related to the mysterious raid of North Korea’s Embassy in Spain in which masked assailants tied up staff, stole computers and fled to the United States, according to two people familiar with the matter.

On Thursday, federal authorities arrested Christopher Ahn, a former U.S. Marine and a member of Free Joseon, a group dedicated to the overthrow of North Korea’s Kim Dynasty. He appeared in a federal-district court in Los Angeles on Friday where his attorney requested that the case be sealed. The court ruled in the attorney’s favor over the government’s objections, a Justice Department spokesman said.

Separately, federal agents raided the apartment of Adrian Hong, the leader of Free Joseon, said people familiar with the incident, who spoke on the condition of anonymity to discuss a sensitive law enforcement issue.

A spokesman with the FBI deferred questions on the case to the Justice Department.

“We will not comment on this particular matter at this time,” said Nicole Navas Oxman, a spokeswoman for the Justice Department.

The developments mark a dramatic turn of fate for the revolutionary group, which sought to assist U.S. authorities by handing over computers and other items stolen from the North Korean embassy that it characterized as potentially having “enormous” intelligence value.

In a statement to The Washington Post, Hong’s lawyer, Lee Wolosky, said he was “dismayed that the U.S. Department of Justice has decided to execute warrants against U.S. persons that derive from criminal complaints filed by the North Korean regime.”

“The last US citizen who fell into the custody of the Kim regime returned home maimed from torture and did not survive,” he said, referring to Otto Warmbier, a U.S. college student who was imprisoned in North Korea in 2016 and died shortly after being flown back to the United States in a coma in 2017.

“We have received no assurances from the US government about the safety and security of the US nationals it is now targeting,” Wolosky said in the statement.

The Justice Department spokeswoman noted that “extradition treaties generally provide that an individual who has been extradited to another country to face criminal charges cannot thereafter be extradited to a third country without the consent of the original country.”

The group asserted responsibility for the raid last month after a judge in Spain lifted a secrecy order in the case and accused Hong and two other men of participating in the incident as a part of a 10-member group.

The brazen daylight raid in February came just five days before President Trump met with North Korean leader Kim Jong Un in Hanoi, and raised questions about whether it was the work of government intelligence services.

The judge, Jose de la Mata, said evidence of various crimes had been found, including trespassing, threats, illegal detentions and burglary committed by a “criminal organization.” The judge said one of the men later shared material stolen from the embassy with the FBI.

The March statement by Free Joseon, also known as the Cheollima Civil Defense group, pushed back against Spanish media reports that the group beat and gagged embassy staff.

“All occupants in the embassy were treated with dignity and necessary caution,” the group wrote. It also claimed that “no other governments” were aware of the raid until after it occurred. Initial media reports in Spain alleged that the CIA was involved.

State Department spokesman Robert Palladino said Tuesday that the U.S. government “had nothing to do with” the incident.

The group’s description of events could not be independently confirmed, but Wolosky said his client can verify its account, including that the Free Joseon members were invited into the embassy.

According to Spanish media reports, the assailants tied up the embassy staff with rope, put hoods over their heads and asked them questions.

More than an hour into the raid, a woman reportedly escaped, and her screams for help alerted a neighbor, who contacted police. When authorities arrived at the embassy, a man opened the door and told them there was no problem. Moments later, the embassy gates opened, and the assailants dashed out to two embassy cars and sped away, according to local reports. The vehicles were found abandoned on a nearby street.

Ahn, who appeared in the U.S. District Court for the Central District of California, was involved in the group’s 2017 evacuation of the nephew of Kim Jong Un from Macau when potential threats to his life surfaced, according to Wolosky. The nephew was the son of Kim Jong Nam, the North Korean leader’s exiled half brother who was assassinated in a nerve-gas attack in a Malaysian airport that same year. Kim Jong Nam was widely believed to have been killed by the regime, making his son a likely target.

The involvement of Free Joseon in the raid was first reported by The Post in March.

Matt Zapotosky contributed to this report.