Up to 90 potential victims of a paedophile who taught at a prestigious London private school are being sought by the FBI.
American William Vahey, 64, worked at Southbank International School from 2009 to last year following stints at a number of other schools around the world.
He was found dead last month and is thought to have killed himself, the FBI said.
Two days earlier, investigators had filed a warrant to search a computer thumb drive belonging to him, which contained pornographic images of at least 90 boys aged from 12 to 14, who appeared to be drugged and unconscious.
It is believed that all of the boys in the images were students of Vahey's, going back to 2008, and that he had sexually abused all of them.
Vahey, who was jailed for child sex offences in California in 1969, typically taught in international schools, which has led the FBI to believe his victims are from all over the world.
The FBI's Special Agent Patrick Fransen said: "I've never seen another case where an individual may have molested this many children over such a long period of time.
"I'm concerned that he may have preyed on many other students prior to 2008."
Vahey was confronted about the images by a colleague at the American Nicaraguan School in Managua, Nicaragua, where he had most recently been teaching, and he confessed that he was molested as a child and had preyed on boys all his life, plying them with sleeping pills before abusing them.
The photos were catalogued with dates and locations that corresponded with overnight field trips that Vahey had taken with students since 2008, but he had led pupils on such outings for his entire career.
Southbank International School's chair of governors is former Ofsted chief inspector Sir Chris Woodhead.
He told the Guardian: "This is a completely shocking revelation. We are co-operating with what is now an international police inquiry."
Vahey, who had a home in London as well as in South Carolina, was found dead in Luverne, Minnesota, on March 21.Suggest a correction