A major incident was declared on 27 September near the UK’s RAF Fairford base, in the Cotswolds in Gloucestershire, after three suspicious vans were seen near the airbase.
No explosive materials were found in the vans, but a quantity of petrol was seized.
The DOJ said that in August, al-Hassani “allegedly directed and paid associates” in the UK to “conduct preparatory steps and arrange logistics for the RAF Fairford terrorist plot”.
On 22 August, al-Hassani paid one individual to “perform pre-operational surveillance of RAF Fairford’s perimeter”, the department said.
In September, al-Hassani paid several people to help him prepare for the plot, the DOJ also alleged.
According to its statement, this included “procuring vans and barrels of fuel, researching the purchase of chemical accelerants, including large quantities of potassium chloride, and purchasing metallic objects that could be used as explosive shrapnel”.
The suspects then gave the vans an appearance of being used by a fuel delivery business and a swimming pool supply business, the DOJ stated.
“On 26 September, al-Hassani allegedly tasked individuals to drive three vans, some of which were later determined to contain barrels of fuel, to the RAF Fairford base.”
The DOJ also said al-Hassani was “in frequent communication” with the van drivers and helped to direct them where to drive.
“At least two of the three vans, while en route to the perimeter of RAF Fairford, encountered a security roadblock and were not allowed to proceed on their intended route,” it added.
It was in the very early hours of 27 September, that the vans were spotted by a local farmer who was heading home from a party, and called the police.
The Iranian embassy in London has said it “categorically rejects and strongly condemns” what it called “unfounded and malicious speculations” linking Iran to the incident.
Two British nationals appeared in court last month in connection with the alleged Iran-linked plot.
The US withdrew all its bombers from RAF Fairford after the incident, with President Donald Trump citing “some threats” to the base as the reason.
RAF Fairford has been used by the US Air Force (USAF) for decades.
In recent months, the US has flown B-1 and B-52 bombers from there to the Middle East for “defensive” strikes on Iran.
In July, Iran said “any base used for aggression against Iranian territory constitutes a legitimate target for our forces”.