LOS ANGELES (AFP) - The German who posed as a Rockefeller family member after allegedly killing a man in California explained the freshly-dug soil in his back yard as plumbing work, a witness said.

Christian Gerhartsreiter, accused of burying the dismembered body of Mr John Sohus in his backyard in 1985, gave the explanation to guests he invited to his house to play Trivial Pursuit.

Ms Dana Glad Farrar said she knew Gerhartsreiter as Christopher Chichester, a self-described British baronet, when she went to his home in San Marino, near Los Angeles, in the summer of 1985.

The guests gathered in the backyard of the guest house he rented, which stood behind the main house occupied by Mr Sohus and his wife Linda, owned by Mr Sohus' aging mother Ruth, who also lived there.

"Well, shortly after I sat down, I noticed there was dirt in the yard that had been dug up," she told the LA Superior Court jury, which heard opening prosecution and defense statements on Monday.

"It looked like someone had dug up part of the lawn ... I said, 'What's going on with your yard, Chris? It's all dug up.' He said he had been having plumbing problems."

Gerhartsreiter, 52, is charged with murdering Mr Sohus, whose remains were only found nine years later when new owners of the house were digging up the yard to make a swimming pool.

Mrs Linda Sohus vanished at the same time, as did Gerhartsreiter - who moved to Connecticut and changed his name a number of times, eventually calling himself Clark Rockefeller and getting married, fooling even his wife for 12 years.

On the day playing Trivial Pursuit, Ms Farrar asked Gerhartsreiter why he went into the main house two or three times to fetch items like sugar and ice, rather than the guest house which was nearer and where he lived.

"He said, 'They are away. They will not mind'," she said, adding that she had doubts about him at the time, particularly because he said he was from South Africa and rich.

"There were a lot of inconsistencies ... I didn't really have any reason to doubt it, but he had a really old car," she said, adding that she never heard from him again, after that day.

Ms Farrar was the second witness called by the prosecution, after the owner of the house who unearthed Sohus' bones in 1994, Mr Jose Perez - who described finding the remains wrapped inside plastic bags and a fiberglass container.

"That's the skull," Mr Perez said, when asked by prosecutor Habib Balian about a photo taken in the yard. He estimated that it would have taken someone six to seven hours to dig the hole in which the remains were buried.

The German's defence lawyer claimed Monday that Mr Sohus' wife Linda could just as easily have killed her husband, noting that she was 6 feet tall and weighed 200 pounds, whereas Gerhartsreiter was 5 foot 6 inches and weighed 140 pounds.

Key evidence could be postcards apparently sent by Mrs Linda Sohus from Paris, three months after the couple's disappearance. Prosecutors say Gerhartsreiter arranged for them to be sent, while his lawyer said she did in fact mail them.

Expected to last three to four weeks, the trial is being held in the downtown LA courtroom where Michael Jackson's doctor Conrad Murray was found guilty in 2011 over the King of Pop's June 25, 2009 death.

The trial continues Wednesday.