Two U.S. senators sent a letter to Delta Air Lines and other carriers raising concerns — and seeking answers — about recent computer outages that caused mass cancellations.

Atlanta-based Delta canceled more than 2,100 flights last week after a computer outage led to the shutdown of the airline’s operations, followed by the disruption of flights for days.

Sens. Edward Markey, D-Mass., and Richard Blumenthal, D-Conn., wrote in a letter to Delta CEO Ed Bastian: “We are concerned with recent reports indicating that airlines’ IT systems may be susceptible to faltering because of the way they are designed and have been maintained.

“Not only may these IT systems be more prone to failure, but they may also be vulnerable to cyber attacks,” the letter reads.

“Now that four air carriers control approximately 85 percent of domestic capacity, all it takes is one airline to experience an outage and thousands of passengers could be stranded, resulting in missed business meetings, graduations, weddings, funerals and other prepaid events.”

The letter mentioned Delta’s outage, Southwest’s outage last month and a United Airlines outage last year.

The senators wrote that they believe airlines should offer to rebook passengers on another airline or other transportation without any additional charge, and should reimburse passengers for any necessary meals and lodging.

The letter asked for a list of IT outages and effects, any safeguards put in place, details on the state of the airlines’ IT system and policies on rebooking and compensation.

Delta said it received the letter and is “in the process of reviewing their requests.”

During last week’s episode, CEO Ed Bastian acknowledged some of the company’s systems were dated but said he did not think that was the root of the problem. He cited a failure to ensure that some of Delta’s computer servers had proper backup, which left them vulnerable when an electrical switch failed and shut off power.

Separately, airline analyst Helane Becker of Cowen and Co. estimated that the effects of Delta’s outage, including cancellations, delays and customers booking with other airlines, will translate into a roughly $120 million hit to Delta’s third-quarter operating income. The financial impact of vouchers for affected customers could stretch over a longer period, since they are valid for about a year, according to Becker.