Major European airports are starting to recover after they were hit by a cyberattack that affected check-in systems, causing flight cancellations and huge delays for thousands of passengers over the past two days.

Starting late Friday, major airports in Brussels, London and Berlin were hit by disruptions to electronic systems that snarled up check-in and sent airline staffers trying options like handwriting boarding passes or using backup laptops. Many other European airports were unaffected.

It was not immediately clear who might be behind the cyberattack, but experts said it could turn out to be hackers, criminal organizations, or state actors.

The cyberattack affected the software of Collins Aerospace, whose systems help passengers check in, print boarding passes and bag tags, and di

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