The National Security Agency — the top U.S. electronic intelligence service — has joined a probe of the October cyber attack on Nasdaq amid evidence the intrusion by hackers was more severe than first disclosed.

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 who attacked and what was taken. It may also show the attack endangered the security of the nation’s financial infrastructure.

The NSA’s most important contribution to the probe may be its ability to unscramble encrypted messages that hackers use to extract data, said Ira Winkler, a former NSA analyst and chief security strategist at Technodyne.

The probe of the attack on the second-biggest U.S. stock exchange operator, disclosed last month, is also being assisted by foreign intelligence agencies, according to an unnamed source.