MADRID (Reuters) - Spain has arrested a 32-year-old Russian computer programmer at Barcelona airport who is alleged to have designed and used software to steal bank account details from banks and individuals, Spanish police said on Friday.

Working with the U.S. Federal Bureau of Investigation (FBI), the man, named Lisov, was arrested by Spanish police on Jan. 13 as he waited to take a flight to another European country. He is suspected of leading a financial fraud network, the police said in a statement.

Lisov, wanted by the United States under an international arrest warrant, had been under observation by authorities for several days in the north-eastern region of Catalonia, police said. Police did not give the man’s first name.

Lisov had been under investigation by the United States for two years for developing and using “NeverQuest,” a computer virus that spreads itself via social media, email and file transfers and has led to the loss of millions of dollars.

An investigation of servers operated by Lisov in France and Germany revealed databases with lists of data stolen from banks, including account balances. One of the servers had files with millions of bank account access details such as user names, passwords and security questions, police said.

The Russian is being held in Catalonia before Spain’s High Court decides whether to extradite him to the United States. The United States has requested his extradition, a police spokesman said.