A dozen tipper lorries valued at £1 million were deliberately set alight, an investigation has shown.
Police have appealed for information into the arson attack at the depot in Great Warley just off the A127 in the early hours of Sunday morning.
A joint police and fire service investigation launched following the fire at a site off Codham Hall Lane, Great Warley, has shown that separate fires were started in the cabs of all 12 vehicles.
The yard, which is close to the Junction 29 interchange between the M25 and A127, is on the former construction site used during the widening of the motorway.
At the height of the fire, 48 firefighters from six fire stations were involved in tackling the fire.
Investigating officer PC Richard Dines said: "The transport yard is close to the A127 and the M25 and we would like to hear from anyone driving past who might have seen any suspicious activity in the area.
"We believe that whoever started these fires might have broken into each lorry to start a fire in the cab which means they could have been in the yard for some time before 12.30am.
"We also want to hear from anyone who might have any information about any events leading to the arson attack."
Anyone who can help should call Pc Dines at Loughton CID on 101 or call Crimestoppers anonymously on 0800 555111.
