WASHINGTON — The U.S. Transportation Department is investigating five of the nation’s largest airlines for potential price gouging in the aftermath of May’s deadly Amtrak crash, the agency said Friday.

The department on Friday sent letters to American Airlines, Delta, JetBlue, Southwest and United asking for information about what they charged customers in the weeks after the crash, when Amtrak service in the Northeast was disrupted.

The Transportation Department, which regulates the airline industry, says travelers complained about “irregular fares.”

When contacted, the airlines said they were cooperating with investigators. American Airlines, Delta and United denied wrongdoing, saying they didn’t raise prices.

In the days after the Amtrak crash, some airlines said they would add flights or fly bigger planes to meet the surging demand, but tickets were selling for upwards of $1,000.

“The idea that any business would seek to take advantage of stranded rail passengers in the wake of such a tragic event is unacceptable,” Transportation Secretary Anthony Foxx said in a statement. “This department takes all allegations of airline price-gouging seriously, and we will pursue a thorough investigation of these consumer complaints.”

The announcement marks a shift for the Transportation Department, which has had fairly limited oversight over the prices airlines can charge.

“We have not investigated an airline for this type of conduct in at least the last 12 years, if ever,” a department spokeswoman said in an email.

The investigation was prompted in part by a letter from Sen. Chris Murphy, D-Conn., who in May asked federal authorities to look into reports of unusually high fares after constituents who travel by train complained that they couldn’t afford a ticket.