Counter-terrorism police are understood to be investigating a potential link with Iran after an alleged bomb threat against a U.K. base hosting the U.S. Air Force was foiled over the weekend.

British police arrested five men on Sunday near RAF Fairford in the west of England on suspicion of explosives and terrorism offenses.

RAF Fairford has been used as the U.S. Air Force’s command outpost for operations from Europe and for American strikes on Iranian missile sites.

In July, Iran’s Revolutionary Guard warned any base used in air strikes on the Islamic Republic was a legitimate target. Iranian ambassador Seyed Ali Mousavi has previously said allowing U.S. bombers to launch from the Fairford base was an “important matter for our self–defense.”

British newspaper The Daily Telegraph reported on Monday that U.K. police are understood to suspect that Iran is the most likely state actor behind the alleged plot.

If Iran is involved in the alleged plot, the Telegraph’s sources said Tehran was unlikely to have relied on criminal proxies as it had in previous incidents. This suggests the potential for more direct state involvement.

Asked about the potential for Iranian involvement in the alleged plot, U.K. Defence Secretary Wes Streeting said on Monday it would be “very unwise” for him to speculate.

Talks between the U.S. and Iran appeared to end in stalemate, with U.S. President Donald Trump rejecting Tehran’s conditional offer to reopen the Strait of Hormuz.

However, Trump told Axios in a phone interview on Sunday that he expected U.S. negotiators to engage in more talks this week.