Peter Wilson's body, which was carefully removed from beneath the sands of a picturesque Co Antrim beach yesterday, had lain there undisturbed for close on four decades.

A vulnerable man with learning difficulties, he was killed by the IRA in 1973. He was just 21. His killers told no one they had done it, or why, simply adding him to their list of "the disappeared". They took him from west Belfast and shot and buried him on Waterfoot beach in north Antrim, which is designated an area of outstanding natural beauty. But the Wilson family have endured 37 years of torment.

Wilson is the ninth "disappeared" IRA victim whose body has been recovered after years in similar untended and unacknowledged graves; the bodies of seven victims have yet to be discovered. The site of Mr Wilson's grave lent an especially poignant dimension in this case, for his family used to go to the beach, oblivious to the fact that he lay buried beneath their feet. A family statement said: "The beach at Waterfoot was a place we have visited often over the years with our mother and children, unaware that Peter was buried there."

Peter Wilson's sister, Anne Connolly, said it was an emotional time for the family. "We were totally shocked," she said. "We couldn't believe it because my mother had sat on that beach many times over the years. But there's a comfort in that, that my mother had been there: in a sense she was with Peter, sitting there.

"Peter was a character," she added. "He was very childish in his ways. Peter basically behaved like a child. He always talked about wanting to join the army, he wanted to be a soldier. That's how naïve he was at that time. In those years, where we lived, you did not join the army."

His parents are now dead, his mother living until she was 90, but he was one of six children. His five siblings are still alive. The discovery coincided with one of the annual events in which relatives of the disappeared come together. Earlier yesterday, another of his sisters, Patricia Gearon, carried a black wreath in an annual silent walk to the Northern Ireland Assembly in Belfast, together with relatives of other victims.

Anne Morgan, whose brother Seamus Ruddy is thought to have been killed by another republican group and buried in France, said: "We carry the symbolic black wreath with the white lilies which represents those who are still missing. Our walk is a reminder that our plight is ongoing and that every effort needs to be made to bring our loved ones home for Christian burial."

Relatives also take part in a yearly remembrance Mass held on Palm Sunday, which, they say, brings comfort to those who continue to struggle with the pain and grief of loss. The pace at which bodies are being recovered has accelerated in recent months, with three victims found since July.

Some "digs" have lasted many months, and have occasionally found no remains, but in Peter Wilson's case, excavations began only yesterday after the ground was prepared last week. This followed an announcement that "reliable and high quality" information had been received.

Most of those who vanished were killed during the 1970s but it was not until 1998, after years of heavy pressure, that the IRA acknowledged that it was responsible for most of the deaths. It said it would co-operate with the authorities, which set up a unique organisation to receive information on where bodies were buried. This operates under legislation guaranteeing that any information passed on will be inadmissible in criminal proceedings.

The disappearances remain in the news on a regular basis in Belfast, often focussing on the Sinn Fein president, Gerry Adams, who, according to tapes left by a now-dead IRA leader, Brendan Hughes, had ordered the killing of Mrs Jean McConville. She was a widowed mother of ten whose case was the first to come to public attention. She was abducted in December 1972 from her home on the Falls Road in west Belfast by a group of 12 republicans and was never seen again. Her body was found in 2003 on Shelling Hill beach in Co Louth.

Mr Adams has denied all knowledge of the killing – and indeed maintains he was never a member of the IRA. The testimony from Brendan Hughes is regarded as powerful evidence, although in later life he became a bitter critic of Mr Adams's. The accusations have certainly had no significant electoral effect on the vote for Sinn Fein, or indeed on Mr Adams's large personal vote.