As the statement from John (right) and Carol Marshall was read out during a press conference, police revealed that Thomas may have been lured to his death by someone he befriended via his CB radio.
The couple's statement said: "Words cannot express the pain we feel at losing the most precious thing we had."
Detectives said that Thomas, from Happisburgh, Norfolk, had regularly used a CB radio at his home and used the call sign "Jackpot".
Thomas had told his parents that he was going to visit a friend in the neighbouring seaside village of Eccles, before he disappeared at around 6pm last Thursday. His body was discovered the following day, 50 miles away, in a picnic area off the A11. He had been strangled.
The couple sat alongside Thomas's uncle, Martin Fiddy, at the 10-minute news conference at North Walsham police station, Norfolk.
None of the family said a word as police spokesman Peter Steward read out their appeal. The three clutched hands tightly.
Mr Marshall, an archivist, looked drained and hollow-eyed, and slumped in his chair, as the appeal was read out.
Mrs Marshall struggled to fight back tears and gripped her husband's hand fiercely.
The statement added: "Thomas was at the start of a life full of promise. Please do not allow another child to be deprived of his future in such a brutal way."
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