The U.K.’s National Crime Agency has arrested a man during its investigation into a cyberattack that disrupted check-ins and delayed flights at several major European airports over the weekend.

The NCA arrested a man in his forties in West Sussex, a county in the south of England, on suspicion of offenses under the U.K.’s Computer Misuse Act, the agency said in a statement September 24. He has been released on conditional bail, it added.

The investigation continues and is in its “early stages,” according to Paul Foster, head of the agency’s cyber unit.

The cyberattack, which began on September 19, affected an important provider of software that underpins check-in and boarding systems, snarling travel at London Heathrow, Dublin, Berlin Brandenburg and Brussels airports through the weekend. 

The attack targeted a software system called MUSE, operated by U.S. firm Collins Aerospace. Hackers used ransomware to lock down the company’s systems and render them inoperable, according to the European Union Agency for Cybersecurity.

“We understand the United Kingdom National Crime Agency has made an arrest and appreciate their ongoing assistance in this matter,” RTX Corp., Collins Aerospace’s parent company, said in a statement. 

RTX separately confirmed it had experienced a ransomware attack in a filing to the Securities and Exchange Commission on September 24, and said it was working with external experts to investigate the breach. The incident hasn’t had a material impact on its financial or business operations, the company said.

With kiosks and bag-drop machines offline, airports have been forced to rely on manual processing, slowing passenger flows and resulting in flight delays and cancellations.

Disruption in London and Berlin had eased by September 22. Heathrow Airport said in a statement on September 23 that most of its flights were departing as normal, but added that it had deployed additional staff in terminals to assist passengers.

Passengers traveling via Brussels Airport have continued to face some delays and cancellations, with the airport advising that there would be “limited disruption” on September 24. The airport has asked passengers to check in online ahead of their scheduled flights.