The United States removed its heavy bombers from a base in the United Kingdom it had used to launch strikes on Iran, amid an investigation into a suspected plot to attack the base.

At least a dozen planes, including B-1 bombers, were redeployed to the United States from Royal Air Force Fairford, according to the Pentagon, which did not specify when or why the planes departed.

“While operational security precluded us from confirming the movement of our assets and forces in real-time, we can acknowledge now that all U.S. bombers that were deployed to RAF Fairford have re-deployed to their home stations in the United States,” a Defense Department official said in a statement on Sunday.

Secretary of State Marco Rubio said Monday morning that the aircraft redeployment reflects a “regular rotation” and that he “wouldn’t directly tie” the suspected plot regarding RAF Fairford to the action.

“[I]t’s not unusual to see rotations happen and movement of assets to different parts of the world based on exercises, redeployments, people changing out and new people coming in, new planes coming in, old planes going out,” Rubio said.

The Pentagon’s statement on the departure came after five men were arrested last month on terror charges near RAF Fairford.

They were arrested in possession of three vans in the village of Whelford, which is about 2 miles from the base, which hosts the U.S. Air Force and has since February been serving as a key hub for American operations against Iran — including for heavy bombers like the B-1.

The five were later released on bail after “extensive interviews,” authorities said.

The suspects, who have not been charged, “remain under investigation as we explore multiple lines of enquiry,”  Laurence Taylor, head of National Counter Terrorism Policing in the United Kingdom, said on Sept. 28.

Prime Minister Andy Burnham told the BBC News in late September that there were “strong indications” that Iran “played a part in what happened.” 

The Department of Defense said in its Sunday statement that the “U.S. Air Force bombers routinely operate around the world to support global force readiness and operational requirements.”

“The Airmen that operate America’s bomber force, including B-1s, B-2s, and B-52s, remain ready to deliver precision global strike capabilities – including through operations that launch and recover from the continental United States – anywhere, anytime,” the statement said.