A field search for a missing farmer could take "a number of weeks" to complete, police in Hertfordshire have revealed.
William Taylor, 70, was last seen at his home in Gosmore, near Hitchin, at about 21:00 BST on Sunday, 3 June.
More than 100 specialist volunteers are scouring fields after police extended the search area to a 1.7km (1.05 miles) radius from Mr Taylor's farm.
His family described his disappearance as "an unbearable situation".
Mr Taylor - whose 70th birthday was on 11 June - told police his Land Rover had been deliberately set on fire in the days before he went missing.
Detectives have refused to be drawn on whether the search teams are looking for a body, saying at this stage the farmer "may have been taken poorly".
The extended search is covering several fields containing mature oat crops surrounding the farm.
Dozens of volunteers from across the country, including Dorset, Wiltshire and neighbouring Cambridgeshire and Northamptonshire, have travelled to Gosmore to support efforts.
However, the hunt has been hampered by persistent hot weather and the height of crops, described in some places as "chin high".
North Hertfordshire Chief Insp Julie Wheatley said the volunteers were carrying out "fingertip and full line searches" in "difficult conditions".
"Specialist trained officers are searching through the middle of the fields just to make sure we haven't missed anything," she said.
"There are a lot of fields and it's going to take a fair amount of time."
Mr Taylor was last seen wearing a pale blue shirt, jeans and black wellies, and may have been wearing a navy-blue boiler suit.
Police said they are "exploring his background and the circumstances surrounding his disappearance".
The investigation into the reported arson is ongoing.