Detectives are also re- examining dozens of cases that involved sudden or suspicious deaths on the island.
Police searched moorland at Mull Hill, near Cregneish, in the south of the island. They fear several bodies may be buried there. They have also contacted the families of two middle-aged men who went missing on the island. Detectives have also reopened the investigation into the only unsolved murder on the island, that of Marjorie Ashton, a pensioner found strangled at her home in Ballasalla in May 1995.
As part of the same inquiry Lancashire police are re- examining files on fatal fires in the Blackpool area that have occurred in the past two years - up to 10 deaths are believed to have been caused by arson.
The Manx and Lancashire forces have appealed for information about more than 100 pieces of stolen jewellery they have recovered during the joint inquiry. Detectives believe the gems - which include necklaces, brooches and rings - might have been stolen during burglaries on the island and in Blackpool.
The fresh investigations came as a result of police inquiries into the deaths of an elderly couple found at their home in Blackpool.
Lancashire detectives have liaised with Isle of Man police officers, who have opened a murder incident room at their headquarters in the capital, Douglas.
A spokesman for the Isle of Man police said: "A number of local deaths are now being re-examined including that of Marjorie Ashton. Some other deaths are also being very closely re-examined and these inquiries are likely to take some time."
Although officers intended to examine dozens of files of unexpected deaths for clues, only a "handful" were believed to be suspicious.
A spokesman for Lancashire police added: "We are looking at unexplained fires in Blackpool in which people died."
A man appeared before Blackpool magistrates on Monday, charged with the death of Dorothy Harris, 68, in a house fire in Ballasalla on the Isle of Man in February 1996.
Earlier this month, the same man was charged with the murders of Eric Boardman, 76, and his wife, Joan, 74, who were found dead at their home in Bispham, Blackpool.
He is also charged with killing his former landlady, Jemimah Cargill, 75, who died in a house fire in Blackpool on 2 October.
The man, who moved to Blackpool from the Isle of Man two years ago, faces four murder charges and is to appear before magistrates again in the town on 17 November.