Detectives have launched a murder investigation a month after a man disappeared within hours of leaving his home in Scotland and driving across the border.
Paul Taylor, 56, from Annan, Dumfries and Galloway, was last seen as he travelled to Carlisle on 17 October.
Police Scotland had said officers were concerned for his welfare.
But on Wednesday Cumbria Police confirmed its inquiry had been upgraded to a murder probe.
Officers said two 19-year-old men, from the Carlisle area, had been arrested on suspicion of murder and remain in police custody.
The force added a 37-year-old man, from Carlisle, and a 17-year-old boy, from Appleby, were also in custody after they were arrested on suspicion of assisting an offender.
Mr Taylor was driving a blue Vauxhall Corsa - registration PY18 DYO - when he left his home at 21:45 and travelled along the southbound M6.
Police believe the car then headed in the direction of the Kingmoor South area of Carlisle, at around 23:35, and also the Carlisle West area.
Officers are examining potential sightings of the vehicle made in the Yewdale, Sandsfield Park and Brough Road areas in Carlisle on the night and early morning of the 17/18 October.
The car was found abandoned in Langwathby at about 08:45 on 19 October.
Det Chief Supt Dave Pattinson, of Cumbria Police, said: "We have been working round the clock to try and find Paul since he was last seen on Tuesday 17th October.
"As the investigation has developed, and due to new information we have received, we are now treating the disappearance of Paul Taylor as a murder investigation.
"We are still requesting the assistance of the public to provide any information to us which may assist police, so we can understand the circumstances and events of what has happened."
Det Chief Supt Pattinson added: "Don't worry if you think the information is not important or may not be significant, it could be vital to help us establish what has happened to Paul and give his family the answers they deserve."