No Bombs Found in U.K. Case, but Officials Say Threat to Bases Is Real The prospect of an attack on an air base used by U.S. forces prompted stepped-up protections across Europe, but confusion shrouds the case, in which the suspects were released on bail.

Two days after arresting five men on suspicion of a possible terrorist plot near a British air base that hosts U.S. bombers, the police in Britain said on Tuesday they had found no explosives in the suspects’ vans, just gasoline.

At the same time, American officials and their allies portrayed the episode as a serious threat. President Trump has said the men had intended to do “big damage.”And privately, senior U.S. and Western officials said they picked up intelligence in recent days about a threat from Iran, and stepped up security at military installations across Europe in response, including at R.A.F. Fairford, the British base.

The potential ambiguities — were the men Iranian operatives, petty gas thieves, or both? — added to the confusion surrounding the arrests that the authorities have yet to fully explain.

The men were freed on bail on Monday, suggesting another possibility that the officials pointed to: that they were low-level, perhaps unwitting operatives connected to broader Iranian plans involving possible attacks against American military bases across Europe. Iranian operations routinely employ such crews.

British law enforcement agencies have not publicly linked the events near the base to a foreign government. The officials who described the Iranian threat spoke on condition of anonymity because they were not authorized to discuss sensitive intelligence matters during an ongoing investigation.

A major Iranian attack against a high-profile American target inside Britain would be a dramatic escalation in apparent retaliation for the seven-month American and Israeli war against Iran. In July, Iran threatened an attack against R.A.F. Fairford, where the British government has given the United States permission to conduct so-called defensive strikes against Iran using long-range aircraft. The Iranian Embassy in Britain dismissed any suggestion that its government was involved in any possible attack on the British base.

“The Embassy of the Islamic Republic of Iran in London categorically rejects and strongly condemns the recent unfounded and malicious speculations published by some British individuals and media outlets attempting to link Iran to arrest of five suspects in the vicinity of R.A.F. Fairford base,” it said Monday in a statement on Many questions about the developments at R.A.F. Fairford remain unanswered.

Officials have not released the names of the arrested men, in line with British policing practice. It was unclear what steps British and American officials were taking to identify the people in charge of the plot. And the officials briefed on the recent threats said they had yet to determine what the men, who were initially detained on suspicion of explosives violations and planning a terrorist act, were trying to accomplish.

Secretary of State Marco Rubio said on Monday night that the matter at the air base was a “very serious situation” that “clearly involves the hands of a foreign actor” but declined to say whether Iran was responsible. He added, in a television interview, that “we are dealing with elements in the world and in the case of Iran, for example, who have openly threatened to attack American interests globally. We’re going to always take that very seriously.”Officials said that American and British intelligence services worked with Israel and Western countries to discover the threats against U.S. bases, including ones in Germany.

Local residents who live near the British base have said they noticed dramatically increased security at its entrances in recent days. Mr. Trump’s comments added even more uncertainty about what had happened.

On Sunday, he said of the five men that “we had them under view for a long time and we got them.”But the police response suggested that the men had not been under lengthy surveillance. Typically, officers would make arrests well before any surveillance targets potentially armed with explosives approached a high-security target like a military base.

In Tuesday’s statement, Laurence Taylor, who leads Britain’s counterterrorism police, responded to questions about why they had released the men, saying it was an “investigative decision, and one which is grounded in experience and careful consideration of policing powers.”He also said that investigators had searched properties in London, where the five men, all British nationals in their early 20s, were from.

The police said the men would be closely monitored while free on bail. “Often this threat becomes visible to us as criminal activity, often at a small scale and committed by individuals whose primary aim is not terrorism but a quick financial gain,” Mr. Taylor told reporters on Monday.

Counterterrorism experts said the unfolding events raise the possibility of a number of scenarios. For example, the Iranians could be attempting to create a diversion.

They might also be testing the base’s security, said Carlos T. Fernandez, a former top counterterrorism agent in the F.B.I.’s New York office. Though Britain owns the base, about 90 miles west of London, the United States has used it for decades to deploy heavy bombers from its long runways.

Britain has repeatedly called out the Iranian threat to the country and pinned attacks there on Iran. The powerful Islamic Revolutionary Guards Corps has long targeted critics of the government but its efforts took a more violent turn after the start of the war.

An Iranian proxy called the Islamic Movement of Companions of the Right, or Harakat Ashab al-Yamin al-Islamiyya, targeted Jewish communities, journalists and Israeli interests in Britain and across Europe from March to May. It claimed responsibility for seven attacks in Britain.

In May, U.S. federal prosecutors in Manhattan charged a commander of Kataib Hezbollah, an Iraqi militia that is a proxy for the Revolutionary Guards, with leading the attacks in Europe and taking part in an Iranian campaign of retaliation for the U.S. and Israeli assault.

Britain has criticized Iran in connection with those attacks and taken steps to designate the Revolutionary Guards as a terrorist organization.

Philip Kaleta contributed reporting.

Michael D. Shear is the chief U.K. correspondent for The New York Times, covering British politics and culture and diplomacy around the world.