NEW YORK, New York - The US Transportation Department has launched an investigation into possible price gouging while train service was disabled between New York and Washington by the five largest US airlines.

The New York-Washington rail service operated by Amtrak was suspended following a derailment in Philadelphia in May, which killed 11 people and injured 200.

The US regulator has reportedly sent letters to each airline asking them to detail average fares along the route before, during and after the crash, with an explanation for price increases, if any.

The department has also asked the airlines whether they communicated with each other about those fares, which might signal collusion.

"These airlines have allegedly raised fees beyond what you would ordinarily be expected in the Northeast Corridor at a time when the Amtrak line was shut down," Transportation Secretary Anthony Foxx told reporters.

The review involves Delta Air Lines, American Airlines, United Continental Holdings, Southwest Airlines and JetBlue Airways, all five have confirmed they are cooperating with the investigation.

American and United said a statement they were confident that no wrongdoing would be found, while Delta said it actually lowered some of its highest fares by nearly 50% following the crash.

Delta and American said they added seats to accommodate more passengers, and Delta allowed previously ticketed Amtrak customers fly for free.

The review comes on top of a Justice Department investigation into whether the airlines worked together illegally to keep fares high by signaling plans to limit flights.

Consumers are dissatisfied with some of the carriers, which recently squeezed more seats onto planes and added new ancillary fees, increasing the cost of air travel.

The Transportation Department has authority to stop any illegal practice by the airlines, but it is not clear what punishment could result from the investigation.