The FBI has asked for the public's help to identify at least 90 potential victims of a suspected child predator who worked at 10 American and other international schools abroad, including one in the UK.
William James Vahey, 64, killed himself in Luverne, Minnesota, on March 21, the FBI said.
That was two days after agents in Houston, Texas, filed for a warrant to search a computer thumb drive that belonged to Vahey, a US citizen with residences in London and Hilton Head Island, South Carolina.
An employee of the American Nicaraguan School in Managua, where Vahey had recently taught ninth-grade world history and geography, gave the drive to the US Embassy there.
Previously, Vahey had taught at the elite Southbank International School in London between 2009 and 2013.
The storage device contained pornographic images of at least 90 boys, ages 12 to 14, who appeared to be drugged and unconscious, the FBI said.
The agency's spokeswoman in Houston, Special Agent Shauna Dunlap, said that investigators suspect all of the boys in the images were students of Vahey's, going back to 2008, and that he had molested all of them.
A statement posted on the FBI website said: "When confronted about the images by a school administrator, Vahey confessed that he was molested as a child and had preyed on boys his entire life, giving them sleeping pills prior to the molestation."
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 students on such trips for his entire career, the FBI said.
FBI Special Agent Patrick Franson said in the statement: "I'm concerned that he may have preyed on many other students prior to 2008.
"I've never seen another case where an individual may have molested this many children over such a long period of time."
The director general of the American Nicaraguan School, Gloria Doll, said that Vahey was "famous for wanting to lead student experience trips". She said that is why several of his previous schools hired him.
Ms Doll said that, as far as she knew, none of her students had been molested. Vahey started teaching at her school last August but had not led any trips for her school before the allegations surfaced.
She said he had been due to take a group to a model United Nations event in the Dominican Republic the very next day.
"We realised we had probably been spared what was happening," Ms Doll said.
Ms Doll said Vahey's flash drive had been taken by his domestic employee, who returned it to the school and urged officials to take a look.
The FBI said Vahey was jailed for child molestation in California in 1969, but Ms Doll said his arrest did not turn up when the Nicaraguan school checked his criminal background. She said he came highly recommended and had been "probably one of the finest teachers" at her school.
Vahey's career included posts at the Tehran American School in Iran from 1972-73; the American Community School of Beirut in Lebanon, 1973-75; the American School of Madrid in Spain, 1975-76; the Passargad School in Ahwaz, Iran, 1976-78; the American Community School in Athens, Greece, 1978-80; the Saudi Aramco Schools in Dhahran, Saudi Arabia, 1980-92; the Jakarta International School in Indonesia, 1992-2002, and the Escuela Campo Alegre in Caracas, Venezuela, 2002-09. He coached boys basketball at several schools, the FBI said.
Such schools typically serve a mix of students from those countries, as well as the children of diplomats, military personnel and other citizens working abroad, and the FBI said it believes his victims are multinational.
The FBI is concerned that some victims might not even know they were abused.
Mr Franson said: "He had access to children because of his position of trust.
"He created a system that gave him the opportunity and the means to molest children. The manner in which he committed these acts - while the boys were unconscious - may have inhibited them from knowing what happened, making it impossible for them to come forward at the time of the molestation."
The FBI's Houston office is leading the investigation because its squad for crimes against children often goes to Latin America to assist with investigations, spokeswoman Ms Dunlap said.
Vahey's death did not stop the investigation because the FBI is obligated to reach out to the potential victims, she added.
FBI officials declined to say why they believe Vahey went to Luverne, a town of about 4,700 people about 160 miles south-west of Minneapolis, where he killed himself at a motel.
The FBI spokesman in Minneapolis, Special Agent Kyle Loven, said it was "plausible" that he had relatives there. Vahey is not known to have taught anywhere in Minnesota, he added.