U.S. security agents have arrested the British hacker known for discovering a “kill switch” that nullified a widespread ransomware attack earlier this year.

Marcus Hutchins, a 23-year-old malware researcher who uses the name Malware Tech, was detained by the FBI on Wednesday at the Las Vegas airport, where he was preparing to return to Britain after attending two hacking conferences in the city.

Court documents unsealed on Thursday indicated Hutchins was arrested on hacking charges unrelated to the ransomware attack known as WannaCry.

Reuters news agency reports Hutchins is accused of advertising, distributing and profiting from malware code known as Kronos that stole online banking credentials and credit card data between July 2014 and July 2015.

Hutchins has not made a public statement, but his mother told London’s Telegraph newspaper that she expected to be “rather busy tonight,” trying to find out where her son is being held.

Hutchins became an overnight hero in May after disabling the WannaCry worm, which infiltrated software in hundreds of thousands of computers in hospitals, schools, factories and shops in more than 150 countries. Parts of Britain’s National Health Service were infected, as well as the FedEx delivery company, German rail Deutsche Bahn and Spain’s Telefonica.

The attack first became evident May 12, 2017, and continued over the weekend. By May 15, Hutchins had discovered a so-called “kill switch” that disabled the worm.

The malware operators demanded the owners of the computers pay a fee of $300 to $600 to regain control of their computers.
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