The police were informed yesterday about the incident.
During inquiry, the alleged involvement of Murray Denis Ward, a British national and a frequent visitor to the institute, came to the light, the police said, adding that Ward had been associated with NAB as a regular donor for almost nine years.
Ward, who was subsequently arrested, allegedly inappropriately touched the three NAB inmates on September 2, the police said.
However, a caretaker at the institute, came there and Ward left the place. The three minors narrated their ordeal to the caretaker who informed the authorities about Ward's behaviour.
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The institute then informed the police and the accused was arrested. He was produced before a court and is currently in police custody.
Police initially suspected it to be a case of sodomy.
Police were probing whether he had sexually assaulted any other child at NAB since he was a regular visitor there. They were also questioning staff members whether they had seen Ward misbehaving with any other inmate.
Ward, a native of Gloucestershire, the United Kingdom, had been working with Sterlite Technology Limited in Gurgaon till April.
He had suffered a paralytic attack in February and has been under treatment since then. He was staying alone here while his family is in the UK. Police suspect that he is a paedophile.