By Brian Todd, CNN
The Secret Service has launched a new investigation for what's being called an "immense" and embarrassing security breach.
All sorts of sensitive information was lost in a very public place.
Law enforcement and congressional sources tell CNN the U.S. Secret Service is being investigated for a potentially damaging loss of information.
The data was on two backup computer tapes, which contained very sensitive personnel and investigative information, according to the sources.
It might remind you of the new James Bond movie "Skyfall," where the villains steal a device with top secret information on British agents. But in this case, the sources say, the tapes were left by a contractor on a train in Washington's metro-rail subway system.
The incident occurred in February of 2008, but is now the subject of an investigation by the Department of Homeland Security's inspector-general.
That office is not commenting on why the probe is going on now.
CNN asked former FBI Counter-Espionage Agent Eric O'Neill about the loss.
"Some of the information could cause lives to be at risk, if someone wanted to get at the families of a high-level government worker or someone they perceived as being someone who could work against, say a terrorist cell," said O'Neill.
O'Neill is the agent who took down Robert Hanssen, the FBI official who spied for the Russians.
He's depicted by Ryan Phillipe in the move "Breach."
The Secret Service says no lives were endangered by the 2008 loss and no fraud occurred as a result.
But how did this happen?
According to the sources, the contractor was transporting the two tapes in a pouch from Secret Service headquarters in Washington to a now closed data facility in Maryland.
The sources say the contractor got off a metro train, later realized the pouch had been left behind.
The Secret Service and metro police were contacted, and an aggressive search took place. But one source says the tapes have not been recovered.
In a statement, the Secret Service said "these backup tapes were not marked or identified in any way and were protected by multiple layers of security. They could not be accessed without the proper equipment, applications, and encoding."
[Reporter]: "Why put sensitive information about agents or anything else on a removable disc?"
"Well, part of the reason I think-and once again, this is conjecture, that in 2008 when this occurred, some of the information might have been on removable discs is because that's how they transported information. We've, we have leapt forward in technology since then," said O'Neill.
But O'Neill has his own questions.
"Why did a contractor have it? Why wasn't it chained to his wrist with a handcuff in a case that he would, the second he stood up, think 'I need to grab it,'" said O'Neill.
CNN posed that question to a Secret Service official who didn't answer it directly, but said protocols have been put in place to make sure this doesn't happen again.