Three terror suspects arrested last week may have been part of a plot to target a US Independence Day celebration at a UK RAF airbase.
The event on the weekend of July 4 at RAF Feltwell, Norfolk was suddenly cancelled two days beforehand after a raised security threat level.
The event attracts American nationals from across the country and the Red Arrows were due to perform this year.
Twelve days later three men in their 20s and 30s were arrested - one in Letchworth and the others in Luton.
Scotland Yard said the arrests by their Counter Terrorism Command were part of an on-going investigation into Islamist-related terrorism.
A spokeswoman for RAF Feltwell said: “The cancellation was due to an increased security threat level and at this point is in connection to the arrests.
“At this time, the arrests are being investigated so we cannot comment further.”
No other US Air Force bases in Britain cancelled events on the same weekend.
Col. David Eaglin, 48th Fighter Wing vice commander, said they will continue to work with their UK counterparts to keep local communities safe.
He said: “While we were certainly looking forward to this year’s celebration and hosting the US and UK communities at RAF Feltwell, we have to put public safety first and foremost.”
The men remain in custody after a warrant to hold them for a further seven days was granted by the magistrates on Wednesday July 15.
The Metropolitan Police declined to comment on the plot target.