The mystery figure behind what prosecutors call the biggest financial hack of all time is now in U.S. custody.

Andrei Tyurin, a Russian citizen who is alleged to have performed key cyber-work in a hack of JPMorgan Chase & Co. and several other companies, was extradited to New York on Friday from the republic of Georgia.

In a newly unsealed indictment, he is accused of infiltrating the computer networks of a dozen companies, previously identified as Fidelity Investments, Dow Jones & Co., JPMorgan and others.

U.S. authorities had for years been closely tracking the hacker who led the computer hacks behind a vast array of activities including securities fraud, cryptocurrency, credit-card fraud and fake pharmaceuticals.

Federal authorities in New York laid out most of those details three years ago when describing a breach of a swath of the financial system that created more than 80 million victims. What those court filings left unanswered was the identity of the central hacker. Prosecutors called the extradition of Tyurin, 35, a “significant milestone” in the fight against hacking.

With others cooperating in the case, Tyurin will be faced with a decision about whether to cut a deal or face prosecution, an unexpected turn for a suspect that few believed would be turned over to the U.S. His deep web of contacts in the criminal underground could make him useful in a wide range of investigations, including the hacking of the 2016 presidential election.

In the financial firms’ hack, Tyurin allegedly worked in concert with Gery Shalon, an Israeli who the U.S. accuse of masterminding the scheme. From 2012 to 2015, according to prosecutors, Tyurin purloined personal information about more than 100 million of the firms’ clients by infiltrating corporate computer networks, locating customer databases and exporting profile information to computers overseas.

The information from the hacks was used in stock manipulation, Internet gambling, credit-card fraud and bitcoin money laundering, prosecutors say, allegedly generating hundreds of millions of dollars in illicit proceeds.

Shalon was arrested in Israel in 2015 and shipped to the U.S. the next year. Several other figures in the case have either pleaded guilty or were convicted after trial. But more than two years later, U.S. authorities haven’t brought Shalon to trial in the matter, signaling that he may be cooperating with U.S. authorities.

Other targeted companies included E*Trade Financial Corp. and Scottrade Financial Services Inc.

Tyurin faces charges of conspiracy, computer hacking, identity theft and wire fraud. He is scheduled to appear in Manhattan federal court later Friday.