Lawrence Calder told Maidstone Crown Court that he spotted blood on Michael Stone's jeans and T-shirt , jacket, and tool box as they were driving to Mr Stone's flat in Gillingham, Kent, on 10 July 1996.
Mr Stone, 38, is accused of murdering Dr Lin Russell, 45, and her daughter Megan, six, in Chillenden near Canterbury on 9 July 1996. He is also accused of the attempted murder of Josie Russell, now 11. He denies all the charges.
Calder, who is an inmate at Parkhurst Prison on the Isle of Wight, serving a four-year sentence for an unrelated cheque and credit card fraud, told how he became friends with Mr Stone in June 1996. He said they saw each other virtually every day for several weeks. Often the two would be together from 9am until 6pm, meeting at Calder's home in Chatham, Kent, or his girlfriend, Sheree Batt's, home nearby.
But on 10 July, Mr Stone arrived at Ms Batt's later than usual, at between 10am and 10.30am. "Usually he came in to Sheree's house for a cuppa, but he didn't stay for a cuppa," Calder said. "He wasn't himself."
Instead, the two men drove off together. Calder said: "I was talking to him and then I saw down his groin there were specks of blood and one large area of blood." There was further blood on Stone's faded off-white T-shirt, his jacket and toolbox. Calder did not ask Mr Stone about it and his friend did not volunteer any information.
Under cross-examination from William Clegg, QC, Calder admitted that he had originally lied to the police and told them Mr Stone had been fighting and that they had been together on the day of the murder. "At the end of the day, I wasn't with him. I don't know if he did it, but if he did it I wasn't with him," he said.
Earlier the court heard evidence from another friend, John Porter, with whom Mr Stone and Mr Stone's brother, Peter, had gone driving near to the scene of the Russell murders in the summer of 1991.
"Michael gave the details to Peter when we were driving. Peter was driving and it was Michael who was giving directions. Michael did know the area well," Mr Porter said.
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