WASHINGTON – A mysterious group that calls itself the Shadow Brokers claims to have hacked into the National Security Agency, stolen powerful cyberweapons and surveillance tools, and put them up for auction.

If true, the claim would indicate one of the U.S. government’s key agencies for cyberwarfare is itself vulnerable and has fallen into a pitched and escalating battle with a powerful unknown cyber foe, perhaps Russia.

News of the apparent breach came over the weekend when the Shadow Brokers released files, claiming they were part of an arsenal “made by creators of stuxnet,” and other notorious NSA malware that helped cripple Iran’s nuclear program in 2009 and 2010.

Neither the NSA nor the Office of the Director of National Intelligence responded to queries. But several cybersecurity experts suggested the penetration is part of rising tensions between the U.S. and Russia.

Edward Snowden, the former CIA employee and NSA subcontractor who leaked a trove of secret NSA documents, tweeted that “circumstantial evidence and conventional wisdom indicates Russian responsibility” for the apparent hack, and the theft is a message that a series of tit-for-tats between Washington and Moscow “could get messy fast.”