The department of justice opened a formal criminal probe last week after the matter was referred by the Pentagon, which is assessing the damage done by the release.

Reuters has reviewed more than 50 of the documents, labelled ‘Secret’ and ‘Top Secret’ but has not independently verified their authenticity, and the number of documents leaked is likely to be over 100.

“There’s a full-blown investigation going on, as you know, with the intelligence community and the justice department, and they’re getting close but I don’t have an answer,” Biden told reporters in the middle of a three-day Irish tour.

“I’m not concerned about the leak, I’m concerned that it happened but there is nothing contemporaneous that I’m aware of.”

The person who leaked the documents is a gun enthusiast in his 20s who worked on a military base, the Washington Post reported on Wednesday, citing fellow members of an online chat group.

The Post based its report, which did not name the person, on interviews with two members of the Discord chat group.

Discord said in a statement on Wednesday that it was cooperating with law enforcement.