FBI agents investigating the activities of the alleged hacker Ryan Cleary have flown into Britain.

Cleary, 19, who appeared before magistrates on Thursday, was arrested on Monday as part of an investigation into cyber-attacks in Britain and the US. Hours after his arrest at Cleary's family home in Wickford, Essex, the FBI agents arrived in the UK, it has emerged, which will add to speculation that Washington is considering a request for his extradition.

Cleary has been charged with five offences of hacking that are alleged to have targeted three British based websites. Police and FBI investigations continue.

Cleary appeared at City of London magistrates court charged with a cyber-attack on Monday on Britain's Serious Organised Crime Agency (Soca), attacks on the International Federation of the Phonographic Industry in November 2010, and on the British Phonographic Industry in October 2010.

His arrest was linked to a series of cyber-attacks by a group called LulzSec, which investigators believe had targeted websites including ones belonging to the American CIA, the US Senate and the electronics company Sony.

Because Cleary has been charged in the UK, that case would take precedence over any extradition request from the US.

The FBI, which gives cyber-crime high priority, is expected to be given access to evidence collected by British police from Cleary's computer equipment recovered from his family home. The FBI is expected to attempt to conduct its own questioning of the teenager.

In court, district judge Quentin Purdy told Cleary: "There may be additional charges resulting from the police investigation."

Ben Cooper, defending Cleary, described him as a "vulnerable young man".

The teenager is being detained at Charing Cross police station in central London for further questioning.