A search in Ireland for the remains of a British soldier killed and secretly buried by Irish republican paramilitaries almost 50 years ago has ended without success, authorities said Wednesday.

Robert Nairac, a British Army captain, was abducted by the IRA from a pub in County Armagh, Northern Ireland, in May 1977 while working undercover and attempting to gather intelligence.

The 28-year-old soldier was taken to a nearby forest across the border with the Republic of Ireland where he was tortured and shot dead. The location of his remains has been a mystery ever since, the BBC reported.

Nairac is one of the highest profile of the so-called “Disappeared”: 17 victims of paramilitary violence during the sectarian conflict over British rule in Northern Ireland known as “the Troubles”, whose bodies were buried in secret.

Four, including Nairac’s, are still missing.

Investigators with the Independent Commission for the Location of Victims’ Remains (ICLVR) — an agency tasked with locating the disappeared — began the search in August on farmland adjacent to the border.

The search, the first to have taken place and involving mechanical diggers, began after information was relayed to the ICLVR, the BBC reported. It is believed to have come from a former IRA member.

But it announced on Wednesday that it had been unsuccessful.

“It is bitterly disappointing that the search for Robert Nairac’s remains has ended without success and our thoughts are with the Nairac family, in particular his sisters Rosemonde and Gabrielle,” said a joint statement by Tim Dalton and Rosalie Flanagan of the ICLVR.

“The investigation and search team did everything they could to bring about a successful outcome but clearly more information is needed.”

ICLVR’s lead investigator Jon Hill appealed for further information to aid the search at the site in Faughart, around 55 miles north of Dublin.

“We were searching a relatively small area, less that one acre (0.4 hectares), and we did so because the information we had was credible,” he added.

“It is our experience from other searches that while we have been in the right area the precise location has not been found first time round,” he said.

UK Northern Ireland Secretary Hilary Benn said he was “saddened” by the unsuccessful search.

“My thoughts are with (Nairac’s) sisters, who continue to live with the pain of not having the remains of their beloved brother returned to them,” Benn said in a statement.

The ICLVR previously dismissed rumors that Nairac’s body was disposed of using a meat grinder, the BBC reported. It also said there was no evidence linking him to the Dublin Monaghan bombings or the Miami Showband massacre – attacks carried out by loyalists in the 1970s.