Citigroup's (NYSE:C) longtime head of security has left for Promontory Financial Group.

Joe Petro, a former Secret Service agent who ran Citigroup's security and investigative services unit for 20 years, retired from the bank this month. He is now a managing director at Promontory, the well-connected consulting firm run by former Comptroller of the Currency Eugene Ludwig.

Petro, 68, spent 23 years in the Secret Service, including four years protecting President Ronald Reagan. Sandy Weill, the then-head of what would become Citigroup, hired him in 1993 to oversee fraud prevention, investigations and security for the company.

In an interview Wednesday, Petro said he plans to help Promontory offer similar security services to bank customers dealing with mounting online attacks, as well as traditional fraud and anti-money-laundering concerns.

There's "an opportunity to help banks, particularly smaller banks that don't have the scale or the capacity to do these things for themselves," he said. "We've only begun to see the challenges associated with cyber fraud."

During his 20 years at the bank, Petro dealt with security issues around the world and coordinated Citigroup's response to the 9/11 attacks. In recent years he has overseen the bank's efforts to combat rising cyber-crime and hacker attacks.

Citigroup has replaced him with Brian Parr, the former Secret Service special agent in charge of the New York field office. Petro said he hired Parr last summer, when he was already starting to plan his retirement, in order to smooth the transition.

 "The evolution of Primerica into Travelers Group and finally Citigroup is the legacy of our company. I will always be grateful for being a small part of that legacy," Petro told his bank colleagues in an internal memo sent earlier this month. "There is a great future ahead for you and for CSIS. Brian Parr will take this organization to new levels."

A Citigroup spokesman confirmed the personnel changes.

Petro is the latest big-name hire for the firm, which has a deep bench of former regulators. Earlier this year, Promontory hired Julie Williams, the former longtime chief counsel at the Office of the Comptroller of the Currency. The OCC eventually replaced Williams with Promontory managing director Amy Friend, in a swap that has drawn some scrutiny of the firm's government connections.

Petro got his new job through another sort of connection: former Citigroup CEO Chuck Prince, who introduced him to Ludwig when Petro was starting to plan his retirement from the bank.

Ludwig and Petro "had a nice chat … and two days later he sent me an offer letter," Petro recalled on Wednesday. "It's a very impressive group of people, a very accomplished group of people … and I'm humbled. I'm looking forward to the challenges here."