A US embassy staffer accused of possessing indecent images of children, was detained at his south London flat and flown out of the UK in August, without being first questioned by British police.
The US Embassy confirmed to the BBC that authorities are investigating the allegations and they have reassigned the staff member to the US while inquiries continue.
"The United States government expects all personnel to adhere to the highest standards of conduct, and we take these allegations seriously," embassy officials said in a statement.
They added that they "quickly informed UK law enforcement and took swift action."
It is understood the individual worked at the US embassy in Vauxhall, south west London, and though not a diplomat, he enjoyed the umbrella of diplomatic protection.
He was flown back to the United States on a commercial flight, but their investigations are ongoing in conjunction with UK authorities.
It is also understood Home Office ministers and the Metropolitan Police are in close contact with the US embassy on this matter as investigations continue.
A UK government spokesperson said: "We expect all foreign diplomats in the UK to abide by our laws, in the same way we expect all UK diplomats to do so when posted overseas."
