Fallout from a cyberattack that affected check-in systems at several European airports extended into a second full day on Sunday, as passengers faced dozens of cancelled and delayed flights and airport teams scrambled to limit the damage to travel plans.

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 hand-writing 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 software of Collins Aerospace, whose systems help passengers check in, print boarding passes and bag tags, and dispatch their luggage. The U.S.-based company on Saturday cited a "cyber-related disruption" to its software at "select" airports in Europe.

While departure boards for London's Heathrow and Berlin's Brandenburg airports were showing signs of smoother arrivals and departures on Sunday, Brussels Airport was still facing considerable issues.

Ihsane Chioua Lekhli, a spokesperson for Brussels Airport, said 45 outbound and 30 inbound flights were cancelled on Sunday, more than double the number a day earlier: 25 departures and 13 arrivals cancelled.

The cyberattack affected only computer systems at check-in desks, not self-service kiosks, she said, and teams were turning to alternative backup systems and pulling out laptop computers to help cope with the impact.

It remained unclear when the situation would be fixed, she said: "For now, we have no idea on the timing, we're taking it day by day."

The airports advised passengers to check the status of their flights before traveling to the airports, and using alternative check-in methods.

"Work continues to resolve and recover from Friday's outage of a Collins Aerospace airline system that impacted check-in," a Heathrow statement said. "We apologize to those who have faced delays, but by working together with airlines, the vast majority of flights have continued to operate."

A rolling message Sunday on the Brandenburg Airport's web page said: "Due to a systems outage at a service provider, there are longer waiting times. Please use online check-in, self-service check-in and the fast bag drop service."

Collins, an aviation and defense technology company that is a subsidiary of RTX Corp., formerly Raytheon Technologies, said Saturday it was working to resolve the issue.

"The impact is limited to electronic customer check-in and baggage drop and can be mitigated with manual check-in operations," it said in a statement.

Fallout from cyberattack on check-in systems at 3 European airports continues for 2nd day