The United States knew Iran or its proxies were going to “try and do an attack, possibly on a United States asset in the [United Kingdom],” Secretary of Homeland Security Markwayne Mullin said Thursday of the arrest and release of five men near a Royal Air Force base used by the U.S.

Mullin told Fox News the U.S. “had already been watching” the situation with British officials and the U.S. was “giving them a heads up.”

His comments come after U.K. Prime Minister Andy Burnham said Wednesday he believes Iran “played a part” in last weekend’s incident at a Royal Air Force base in England that hosts U.S. warplanes involved in the conflict with Iran.

“They were also feeding information to us, so this was something we were paying attention to already, we just didn’t know where it was coming from” Mullin said, adding that Iran is always a threat to the United States and its allies.

“This was a little bit more specific, so we were on heightened alert about the direct threat we had here but didn’t know where it was coming from, so when we saw it we were able to act quick,” Mullin said.

Five British nationals from London between the ages of 23 and 25 were arrested early Sunday in possession of three vans near RAF Fairford, police said, which has long been a key hub for U.S. Air Force operations in the Middle East — including in the conflict with Iran.

Police said the men were suspected of terrorism and explosives offenses, prompting speculation that the detainees were planning to target the RAF base, possibly on behalf of a foreign power.

Investigators said a search of the suspects’ vehicles found no improvised explosive devices and authorities said they were released on bail on Monday after “extensive interviews.”

On Thursday, the U.K. Counter Terrorism Policing unit announced it had arrested another man — a dual U.K./Iranian citizen — in connection with the case on suspicion of “preparation of terrorist acts.” It also said a second man had been questioned “under caution” and that it had searched two properties.

Mullin expressed frustration that the five men were released, but “It doesn’t mean these individuals are just running loose. We — as in our partners and us — have eyes on these five individuals. We not going to let them slip through the cracks.”

Asked if he planned any response to the possible attack, President Donald Trump said Wednesday the U.S. is “studying it right now. We’re studying it. We’re studying it very hard.”

Asked about how the U.K. has responded, Trump said, “The U.K. did release a number of people that we caught wanting to blow up a base. We caught them early and I’m trying to figure out if that’s for real, if they actually did it.”

U.K. Prime Minister Andy Burnham said Wednesday that his government believed Iran “played a part” in the possible plot.

“There are strong indications that Iran played a part in what happened over the weekend at RAF Fairford, but of course it remains an ongoing complex and serious police investigation, and of course we are working closely with our colleagues in the United States of America,” Burnham said.

Reacting to those accusations, Iranian Foreign Minister Aragachi said, “I can confirm Iran’s belief that releasing supposed terrorists working for foreign states really says it all. You’re barking up the wrong tree.”