Iran's embassy in London has said it "categorically rejects and strongly condemns" speculation that Tehran was involved in the incident.
A UK probe into a suspected bomb plot at an airbase used by the United States was mired in confusion on Tuesday, after media reported no explosives were detected in the vehicles seized nearby.
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As armed guards maintained a cordon around a nearby village, Washington questioned the decision to release on bail five men arrested near the Royal Air Force Fairford base in southwest England.
Three white vans were discovered near the base on Sunday and the men, all London-based Britons in their mid-20s, were detained.
Armed police were also visible on Tuesday patrolling around the airfield used by US planes to bomb some targets in Iran.
Police have said officers conducted an extensive examination of the vehicles with the help of "military assets" and forensic experts.
They did not disclose their findings. But The Times newspaper and several other UK media outlets have reported that police found no traces of explosives, easing fears that an attack had been foiled but leaving many questions unanswered.
A resident in the neighbouring village of Kempsford, Patricia told the AFP news agency it was "very embarrassing that people around the world thought it was a terrorist attack but in the end no, it's a lot of noise for nothing."
Announcing the release of the five suspects on Monday, the head of UK Counter Terrorism Policing, Assistant Commissioner Laurence Taylor, said the men "absolutely remain under investigation."
He insisted his officers were keeping an "open mind" and considering "every possible angle" in the security scare, including the possible role of "proxies" working knowingly or unknowingly for a foreign state.
Iran's embassy in London has said it "categorically rejects and strongly condemns" speculation that Tehran was involved.
'Serious situation'
Adding to the hazy picture, US President Donald Trump told reporters on Monday he was "surprised" that British police had released the five, adding: "I wouldn't have done that."
Hours later, US Secretary of State Marco Rubio called the incident "a very serious situation...that clearly involves the hand of a foreign actor" but declined to provide further details.
On Sunday, Trump had said the UK and US were investigating the suspects "for a long time" and that they "were looking to do big damage to our fort."
The security scare became public after a farmer living in the village of Whelford near the airbase rang police after seeing the three vans blocking a road and a group of masked men fleeing the scene.
Armed officers arrested the five men on suspicion of explosives offences and, later, "suspicion of preparing a terrorist act" under anti-terror laws.
Security at the base had reportedly been heightened recently.
Britain gave Washington permission in March to use Fairford for "specific defensive operations into Iran" to destroy Iranian missiles at source.
US bombers and other planes have since been seen taking off and landing from the base, the only one on Britain's mainland used for this purpose.
The lack of clarity over the weekend incident has sparked different unverified theories, with some media reporting allegations that the men could even be fuel thieves.
"It's a bit odd they were here in the middle of the night," one local told AFP as she did some gardening in the nearby village of Kempsford.
"If they were true terrorists they wouldn't have released them," she added, asking not to give her name.
Aerial footage of the vans showed they appeared to contain barrels, and at least one was marked with a fake logo and phone number giving the false impression that it was owned by a legitimate diesel delivery firm.
The legitimate firm said in a statement on its website that "neither the vehicle nor any of the individuals involved has any connection whatsoever with Fuel2U", adding it was cooperating with the police.
Iran linked to series of attacks in Europe
Britain didn't join the US and Israel in the Iran war, which began on 28 February, and initially denied Trump’s request to use bases in the UK to strike the country.
But then-Prime Minister Keir Starmer agreed in March to let the US use bases in Britain for what he called “defensive” operations against Iran’s missiles and their launch sites following Iranian attacks on UK interests and Britain’s allies in the Gulf.
American B-1 and B-52 bombers have taken off from Fairford, 160 kilometres west of London, for operations during the Iran conflict.
Iran has said that it considers any base used to attack its territory a legitimate target.
British security officials say proxies linked to Iran have been behind a string of arsons and other attacks in the UK and other countries, many of them targeting the Jewish community and intended to sow fear.
They have previously warned that Tehran could expand its targets in the UK if conflicts in the Middle East continue.
UK Foreign Secretary Ed Miliband said on Monday that the incident is a “reminder of the threats our country faces.”