The National Security Agency, the top U.S. electronic intelligence service, has joined a probe of the October cyber attack on Nasdaq OMX Group Inc. amid evidence the intrusion by hackers was more severe than first disclosed, according to people familiar with the investigation.

The involvement of the NSA, which uses some of the world’s most powerful computers for electronic surveillance and decryption, may help the initial investigators — Nasdaq and the FBI — determine more easily who attacked and what was taken. It may also show the attack endangered the security of the nation’s financial infrastructure.

“By bringing in the NSA, that means they think they’re either dealing with a state-sponsored attack or it’s an extraordinarily capable criminal organization,” said Joel Brenner, former head of U.S. counterintelligence in the Bush and Obama administrations, now at the Washington offices of the law firm Cooley LLP.

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