British Prime Minister Andy Burnham has said there are “strong indications” that Iran was involved in the weekend bomb scare near RAF Fairford, a UK airbase used by the US military.

Burnham made the remark to the BBC on Wednesday. “There are strong indications that Iran played a part in what happened over the weekend at RAF Fairford,” he said. He added that the UK was working closely with the United States and that the probe was “ongoing, complex, and serious.”

Police were called early on Sunday after a farmer reported three white vans and masked men near the base in Gloucestershire. Five men were arrested on suspicion of explosives and terrorism offences. The five men were released on bail.

The statements mark the first official UK suggestion of Iranian involvement. What happens next depends on the police investigation and whether London shares intelligence with Washington.

Iran has denied involvement and released aerial footage of RAF Fairford on Thursday, claiming it showed the base empty. Downing Street dismissed the footage as a “dangerous PR stunt,” and the prime minister rejected Iran’s claim outright in parliament.