U.S. authorities are investigating possible price gouging by five American airlines while train service was disabled between New York and Washington after a deadly Amtrak crash in Philadelphia in May, the Transportation Department said on Friday.
Transportation Secretary Anthony Foxx said the investigation involved Delta, American, Southwest and Jet Blue in the Northeast Corridor.
CNBC later confirmed that a fifth airline, United Airlines, is also under investigation for the same allegations. (Tweet this)
"These airlines have allegedly raised fees beyond what you would ordinarily expect in the Northeast Corridor at a time when the Amtrak line was shut down," Foxx told reporters at a breakfast sponsored by the Christian Science Monitor.
The Justice Department said earlier this month it was investigating whether airlines worked together illegally to keep fares high by signaling plans to limit flights.
Eight people were killed and more than 200 injured in the Amtrak derailment in May. Amtrak traffic between Washington and New York City was shut down for six days as a result.
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Foxx said the investigation being launched on Friday comes after Democratic Senator Chris Murphy of Connecticut contacted the Obama administration about his concerns.
"There were a flurry of concerns raised in the wake of the accident in Philadelphia," Foxx said.
He said letters were sent to the four air carriers requesting information. He did not say how long the investigation would take.
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--CNBC contributed to this report.