Airline flight booking and search engines crashed for nearly 4 hours on Monday

The engines that power a number of airlines' flight search and booking systems crashed on Monday afternoon, preventing customers from making reservations, and in some cases, even search for flights.

The problem was first reported on Downdetector.com shortly after noon Eastern time, with reports beginning to flood in within minutes. Service appeared to be fully restored shortly before 4 p.m. Eastern.

At least American, Delta, United and Alaska were affected, although it was not immediately clear if other airlines — including international ones — were impacted. The airlines were reportedly also unable to complete ticket sales by phone.

The cause appeared to be linked to an outage on ITA, a back-end system run by Google that some airlines use to manage flights and inventory, according to sources familiar with the matter.

The public-facing portal for ITA was also unable to complete flight searches.

Southwest, JetBlue, Hawaiian, and others did not appear to be affected.

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Some online travel agencies and metasearch sites, including Expedia and Orbitz, appeared to still facilitate searches and reservations on the impacted airlines. However, it was not clear whether displayed prices were accurate, nor whether new flight bookings would be ticketed properly.

Other sites, such as Kayak, would showing flight prices and options, but appeared unable to facilitate new reservations.

Travelers flying Monday were not be affected and could use web and app-based services, including check-in, as normal, according to an airline source.

A Delta spokesperson confirmed in a statement that functionality had been restored later Monday afternoon.

"Delta.com and the Fly Delta app are functioning normally after experiencing an issue this afternoon that made it difficult for customers to purchase flights on delta.com, the Fly Delta app, and through our Reservations Call Center," the statement said. "The issue was caused by the failure of technology provided to Delta and multiple airlines by Google. We apologize for any inconvenience this caused."

A spokesperson for Google similarly confirmed that the issue had been fixed.

"Earlier today, a data error impacted our flight shopping software, which prevented airline partners, as well as Google Flights, from showing fare information," the spokesperson said in a statement. "We've implemented a fix and the issue has now been mitigated. We'll continue to monitor to ensure this is fully resolved."