Detectives say an elderly couple found buried in their back garden in Nottinghamshire died from gunshot wounds.
Police have not yet formally identified the pair, but believe they are William and Patricia Wycherley, who lived at the house in Blenheim Close in Forest Town, Mansfield in the 1990s.
A police spokesman said: "A Home Office pathologist has determined that both people died as a result of being shot."
Their daughter and son-in-law have been arrested on suspicion of their murders, according to the Press Association.
Susan Edwards, 55, and her husband Christopher, 57, were arrested at St Pancras International railway station in London on Wednesday evening, said the news agency.
Nottinghamshire Police have refused to confirm the identities of the suspects, other than saying they are aged 57 and 55.
The couple, who are believed to have been living abroad, are being questioned in police custody.
It comes after human remains were found at the house last month. Police went there following a tip-off.
Police have not yet formally identified the bodies found in the gardenMr and Mrs Wycherley moved to the address in 1987 and, according to neighbours, "simply disappeared" in 1998.
Officers have said Mrs Wycherley is believed to have been born in Fulham, west London, and would now have been 79. Her husband would have been 100.
Police say bone analysis has shed further light on the remains.
One of the bodies is believed to be that of an elderly white man who was older than 60, well-built and around 5ft 8in.
The other is thought to be that of a well-built white woman, who was 40 or older and around 5ft 6in.
Detective Chief Inspector Rob Griffin, who is leading the inquiry, said: "Bill and Pat, who would be 100 and 79 respectively if they were to still be alive today, were known to be quite reclusive.
"But with help from relatives and old friends, we are starting to build a picture of their lives and, more importantly, their movements in 1998, when neighbours say they simply disappeared."