Detectives investigating the disappearance of Corrie Mckeague have started a search of a landfill site.
The 23-year-old airman, from Dunfermline in Fife, vanished while on a night out with friends in Bury St Edmunds, Suffolk, in September.
A bin lorry collected refuse a short time later from the area the RAF Honington gunner was last seen.
Suffolk Police said trawling through the landfill site in Milton, near Cambridge, could take up to 10 weeks.
Officers will search more than 920 sq m (1,100 sq yd) of waste to a depth of 8m (26ft).
Det Supt Katie Elliott said: “Teams of specialist, search-trained police officers from both Suffolk and Norfolk will be carrying out the work to find anything that may be linked to the investigation.
“We need to find Corrie and discover what happened to him. While the search may not provide the answers as to what happened, it is something we need to do as our investigation continues.”
A “needle in a haystack”: Searching a landfill site
Steve Gaskin, a former detective with experience of carrying out large-scale searches during his time with the Metropolitan Police, described this latest operation as a “mammoth task”.
He said officers will need to sift through all the earth that is moved by hand, “because it’s not just Corrie they’re looking for”.
It would also be “any other associated evidence”, such as his jacket, or “anything that may have come out of his pockets”.
He said: “They’ve also got to make sure any forensic evidence that’s accrued is treated and dealt with properly… in case there’s a criminal case.
“There’s been a lapse of six months [since Corrie Mckeague went missing] and you can imagine what’s on a rubbish dump – birds, rodents – so there’s a good chance, with the weather and all the conditions, particularly if he’s a number of metres down, that there will be an element of decomposition.
“This is a mammoth task and it is analogous, I would say, to looking for a needle in a haystack.”
Mr Mckeague was last seen at about 03:25 BST on 24 September.
Police seized a bin lorry in the early stages of the investigation, but no traces of Mr Mckeague were found in the vehicle and the landfill site was not searched at the time.
However, its route appeared to coincide with the movements of Mr Mckeague’s phone.
Mr Mckeague’ mother Nicola Urquhart previously told the BBC: “Common sense suggests that the most likely place Corrie ended up is the landfill site or the incinerator.”
Some 8,000 tonnes of bulk material has been removed from the site to allow officers to safely access the search area.
A 26-year-old man was arrested on suspicion of attempting to pervert the course of justice on 1 March. He has been bailed until 13 April.