Delta Airlines passengers from around the world shared photos and videos early Monday, Aug. 8, after a massive computer failure that caused the cancellation of hundreds of flights. (Monica Akhtar/The Washington Post)

Behind each of the four big players in the U.S. airline industry — American, United, Delta and Southwest — there is a tangled computer system pieced together after decades of mergers that married mismatched networks.

Just what went wrong with Delta Air Lines’ system Monday that caused hundreds of cancellations and delays is still being sorted out. Was it a power outage, as Delta says? Or was it more likely an internal computer glitch, as Georgia Power, the utility at Delta’s Atlanta hub, says?

In either case, aviation and computer specialists say, it shouldn’t have happened. Computer systems and their electric power sources should have foolproof backups, they say.

Tens of thousands of passengers were delayed as Delta canceled 451 of its nearly 6,000 daily flights by mid-afternoon Monday. The airline warned that more cancellations and delays were likely to follow as it struggled to revive its computer network.

But there may be more of the same airline problems in the months to come as long-term reverberations follow a tumultuous decade of mergers that swiftly consolidated the industry.

Delta passengers wait in line at a ticket counter at Newark Liberty International Airport in Newark, N.J., Monday, Aug. 8, 2016. Delta Air Lines delayed or canceled hundreds of flights Monday after its computer systems crashed, stranding thousands of people on a busy travel day. (AP Photo/Seth Wenig) (Seth Wenig/AP)

“These [remaining] airlines have long histories: 20 years of cobbled-together systems,” said Rick Seaney, creator of FareCompare.com, a travel website. “It’s difficult to integrate older technology. They have hundreds and hundreds of cobbled-together things that they have to track down to make sure everything is working correctly.”

Before Monday, the most recent evidence of airline computer system problems came last month, when Southwest Airlines needed several days to fully recover from a collapse in its systems that resulted in more than 2,300 cancellations and 8,000 late flights. Last summer, a United Airlines computer glitch forced the cancellation of dozens of flights and delayed hundreds more.

Southwest absorbed a new and different computer system when it acquired AirTran six years ago. Southwest is investing millions of dollars in the creation of a new computer system to replace its aging network.

American Airlines, which finalized its merger with US Airways last year, is working hard to marry the two systems.

“American’s about to, in the next month-and-a-half or two, start to integrate US Airways systems,” Seaney said. “Typically, when you’re trying to do a cut-over like that, you pick a weekend when you have slow traffic just in case you have glitches.”

Since the turn of the century, American has absorbed TWA. US Airways merged with America West, then with American. Delta merged with Northwest. United merged with Continental.

A Delta Airlines employee hands out snacks to passengers waiting to check in following a Delta Airlines system-wide computer breakdown, at Newark International Airport in Newark, New Jersey, on Aug. 8, 2016. (REUTERS/Joseph Ax) (Staff/Reuters)

Bijan Vasigh, a professor at Embry-Riddle Aeronautical University in Florida, said it may be time for airlines to invest in totally new computer systems to replace the patchwork networks now in place.

“The cost [of Monday’s shutdown] should be huge for Delta Air Lines,” he said “I’m sure that they will try to revisit this issue.”

Airlines rely on their computer systems to manage a number of operations, including obvious ­elements, such as reservations and ticketing, and behind-the- scenes management of plane movement, gate assignments, air crew scheduling and even the displays on arrival-departure screens at many airports.

Delta’s computer system failure came at the height of the summer travel season and on a Monday when many business travelers head to airports to begin their workweek. But Vasigh pointed out that it could have been worse.

“Imagine if this event had happened during Thanksgiving or Christmas; this would be catastrophic for the airline,” Vasigh said. “Recapturing those passengers and accommodating them would be a nightmare.”

But the failure of computer software or the loss of electrical power should not cripple an airline, specialists agreed.

“Anytime you have a systemwide outage, it either has to be one of the core systems or some sort of networking glitch,” Seaney said. “All computer systems have redundancy. Typically, almost all companies — especially if you have credit card data — are required to be spread out, sometimes across different countries, to make sure that basically [their network] never goes down.”

They also have backup power, he said.

“If you have any sort of critical infrastructure, you have gasoline-powered backups for at least 24 hours,” Seaney said. “Typically, what you have is two or three hours of battery backup and another 24 to 48 hours of gasoline-powered backup.”

Delta said its computers stopped working at 2:30 a.m. Monday. Shortly before 9 a.m., the airline lifted a self-imposed ground stop and began resuming limited operations, but it told passengers: “If your flight is canceled or significantly delayed, you are entitled to a refund. Even if your flight is not canceled, you may make a one-time change to your ticket without fee.”

In a later statement, the airline said, “We are aware that flight status systems, including airport screens, are incorrectly showing flights on time.”

Ed Bastian, Delta’s chief executive, issued a video apology to the airline’s passengers, saying people were working “around the clock” to restore operations.