Disruptions caused by a cyberattack on check-in systems at several major European airports dragged into a second day on Sunday, leaving passengers facing dozens of cancellations, long waits, and mounting frustration.

The outage, which began late Friday, hit airports in Brussels, London, and Berlin, forcing airline staff to fall back on handwritten boarding passes and backup laptops. Other European airports remained unaffected.

The problem was traced to Collins Aerospace, a U.S.-based aviation technology firm whose software supports check-in, baggage tags, and dispatch systems. The company confirmed a “cyber-related disruption” and said teams were working to restore normal service.

While London’s Heathrow and Berlin Brandenburg airports reported smoother operations by Sunday, Brussels Airport continued to struggle, canceling 45 outbound and 30 inbound flights—more than double the number scrapped the previous day.

Brussels Airport spokesperson Ihsane Chioua Lekhli said only check-in counters were hit, not self-service kiosks, but admitted there was no timeline for a full fix: We’re taking it day by day.

Passengers were urged to use online and self-service check-in whenever possible and to confirm flight status before heading to the airport. Heathrow apologized for the delays but stressed that most flights were still operating. Berlin Brandenburg advised travelers to expect longer waits and lean on digital check-in options.

Cybersecurity experts said it was too early to determine whether hackers, criminal groups, or state-backed actors were behind the attack.