The body of a missing mother-of-three is believed to be buried or concealed along a 19-mile (30km) stretch of rural road, say police.
Rania Alayed, 25, was reported missing from her home in Cheetham Hill, Manchester, on 2 July, having been last seen in public on the evening of 7 June.
It is thought she was taken in a white camper van in the early hours of 8 June to the Thirsk area of North Yorkshire, where her body was disposed of.
The Leyland DAF 200 vehicle, registration number F864 RJB, is believed to have stopped in an unidentified layby on the northbound A19.
Two men have been charged with Ms Alayed's murder but detectives want to trace her body as their investigations continue.
Officers from Greater Manchester Police (GMP), supported by teams from North Yorkshire and aerial assistance from the RAF, have been searching the A19 between the junctions of the A168 and the A172 at Osmotherley in recent weeks.
Letters have been sent to 350 motorists who are known to have driven in the area between 3am and 5am on 8 Juneto see if they have any information on the movements of the camper van, which has since been recovered.
Ms Alayed previously lived in Norton, Stockton-On-Tees, and moved to Manchester in January.
Her disappearance was described by police as "extremely out of character".
The Independent Police Complaints Commission is also investigating how GMP dealt with previous contact with Ms Alayed, who reported being a victim of domestic violence.
Detective Chief Inspector Peter Marsh, of GMP, said: "We believe Rania is on that stretch of road.
"We are now satisfied that from the 8th of June there is nothing to suggest that Rania is alive.
"It is a huge search and it is a case of narrowing it down. The area is farming land with open fields and ditches around.
"We believe the Rania's body is buried or concealed somewhere.
"We are hoping to hear from anyone who saw this white camper van and, in particular, if they saw it parked by the side of the road or in a layby.
"I am also interested to hear from anyone walking in the area or working who may have seen some suspicious activity or come across any unusual areas of ground such as a disturbance, cut vegetation.
"We have been working closely with our colleagues in North Yorkshire Police and have used specialist search equipment supplied by the military to help locate her.
"As Rania still remains missing, it is crucial that we go to the next stage and publicly appeal for information. If you believe you saw Rania after 7pm on 7th June 2013, we are keen to hear from you."
Anyone with information on the movements of the white camper van should contact the Major Investigation Team on 0161 856 4711.
The two men accused of Ms Alayed's murder are next due in court in October.