Pittsburg police said Thursday that they are cautiously weighing a claim that marijuana found in the home of a slain PC World senior editor was grown and used by his son for medical purposes.

Four masked men burst into the Pittsburg home of Rex Farrance, 59, about 9 p.m. Tuesday, fatally shooting him and pistol-whipping his wife after demanding money, police said. No arrests have been made.

Police summoned to the home on Argosy Court found a large quantity of marijuana and said the couple were involved in the possession and possible sale of drugs. Colleagues of Farrance, a well-respected technical journalist and fitness enthusiast, expressed deep skepticism, and the couple's son said that he was growing the drug at the home for medical reasons.

Pittsburg police Inspector John Conaty declined to say Thursday whether the drugs found at the home were legal, adding that it premature to say whether anyone connected to the home could face drug charges.

As for the family's insistence that the drugs consisted only of medical marijuana, Conaty said, "They have made that claim to us, and we are exploring whether there's any validity to that claim."

Conaty said the motive for the incident was definitely robbery, and whether any drugs on the premises are related to the home invasion "feeds right back into the entire reason for exploring whether there's a nexus between the possession of a controlled substance and targeting that particular residence for residential robbery."

Sterling Farrance, 19, denied that his father and mother, Kaiser Permanente registered nurse Lenore Vantosh-Farrance, 56, were involved with and possibly dealing drugs from the home they've shared since 1991 on a quiet cul-de-sac.

He said he grows medical marijuana at his parents' home with his father's permission and uses it to combat pain from a car crash. He said he and a friend have a doctor's prescription and medical-marijuana cards from the Oakland Cannabis Buyers' Cooperative.

"Somebody thought there was more going on here than there was," he said. "They came in asking for money. They came in expecting to find money."

Sterling Farrance said he moved out of the house a few months ago and has been splitting time between his house in Antioch and his parents' home. Outside the residence Thursday, a memorial of flowers, burning candles and a brown teddy bear was set up.

On the garage door were hand-drawn signs reading, "We love you Rex" and "You will be missed."

"My dad was the most generous person that you could possibly know," Sterling Farrance said. "He always put everyone else before him."

He said of the assailants, "Everyone is trying to rack their brains, trying to figure out who these people might be."

Ed Albro, executive features editor at PC World in San Francisco, said Thursday that any links between Rex Farrance and drugs "just seems like science fiction. For these things to be true, it would have to mean that Rex would have had to completely change his character the moment he stepped out of this office."

Joseph Elford, an attorney for Americans for Safe Access, a national advocacy group in Oakland, said Thursday that police often assume that marijuana they find is solely for recreational use. In 1996, California voters passed an initiative legalizing marijuana for medical use with a doctor's approval.

"Medical marijuana patients typically use a lot more marijuana than a recreational marijuana user," Elford said. "The problem is, the police are still in the paradigm of enforcement that involves recreational use."