A reward in excess of £5,000 is being offered for the safe return of a £3m Henry Moore sculpture.
Henry Moore's Reclining Figure is considered a "national treasure"
The Henry Moore Foundation announced the offer on Monday, as police began to question householders in the area close to where the statue was last spotted.
The bronze Reclining Figure was stolen from the the Perry Green estate in Much Hadham, Hertfordshire, on Thursday.
It was last reported being seen on the back of a Mercedes lorry in Harlow, Essex, within an hour of the theft.
Hertfordshire Police fear it could be melted down and sold as scrap.
The Mercedes vehicle, which had been stolen from Royston about an hour before the theft, was located in Coopersale near Epping, Essex.
It is now undergoing forensic examination.
CCTV cameras filmed the three raiders as they used a crane to lift the two-tonne piece onto the lorry.
Melted down as scrap the sculpture would only have a value of about £5,000.
The Henry Moore Foundation, which owns the sculpture and has offered a "substantial" reward for its return, has begun a review of its security.
Chief Inspector Richard Harbon said police classed the sculpture as a "national treasure" and detectives from Hertfordshire were working with the Metropolitan Police's Fine Arts Squad.
He added: "It is a nationally-renowned sculpture and very, very difficult to get rid of.
"This is not opportunist theft. These are people who knew what they were doing, knew what they were after. It was a very, very audacious theft."
A second vehicle used in the theft was a Daihatsu four-wheel drive with spotlights at the front.
Foundation spokesman Gareth Spence said: "It is quite a daring thing to do, and it will cause a reassessment of our security process.
"Obviously, we are very upset and disappointed.
"The theft alone has caused a great deal of upset, but the thought of it being melted down is worse."
Considered by many to be the most outstanding British sculptor of the century, Moore died at 88 in August 1986.