The Northern California man suspected of killing two women and four children pleaded not guilty Thursday to murder charges in one of the deaths.

Martin Martinez, 30, has been formally charged with killing his girlfriend's 2-year-old toddler last October. He was also a suspect in the Saturday homicides of girlfriend Dr. Amanda Crews, their 6-month-old daughter, her 6-year-old daughter, his mother and an unidentified 5-year-old girl. All of the deaths occurred in the homes Martinez shared with Crews, 38.

Police haven't disclosed the cause of death for the five people killed in Modesto Saturday or discussed possible motives. Martinez isn't charged with their deaths, but police say he's their prime suspect.

Martinez has been charged with murder and child abuse leading to death for the homicide of Christopher Ripley, 2, his girlfriend's son left in his care.

The toddler died of head injuries suffered in a fall. Martinez told police the injuries were caused by an accident. But police allege in court papers that Martinez gave an inconsistent account of how the toddler died.

A pathologist who examined the child's body told investigators Thursday the boy's death appeared to be a homicide.

Modesto Police Chief Galen Carroll said authorities didn't arrest Martinez sooner because they were awaiting the pathologist's report and still gathering other evidence.

"You only have only one shot in making an arrest or a case,'' Carroll said at a press conference this week. "So you want to make sure you do it properly.''

Police on Saturday night obtained a warrant for his arrest hours after discovering the five bodies.

Martinez was arrested in San Jose early Sunday morning while leaving a movie theater with his father. Police won't say how the five people were killed Saturday or discuss possible motives.

Martinez lived with Dr. Amanda Crews, 38, in Modesto until Stanislaus County social workers obtained a protective order related to the toddler's death that forced Martinez to move out of the house about two months ago. The police and the county Community Services Agency declined to discuss details of the order meant to protect Crews' surviving children from Martinez.

Carroll said the couple appeared to have an ongoing relationship after the toddler's death. Crews' stepfather Ron Beidleman told the Modesto Bee newspaper that the couple had broken up but were working to get back together after the boy's death.

"They were trying to reconcile,'' Beidleman said. "But it was not going well.''

Martinez remains jailed in Modesto, and it's unclear whether he has retained a lawyer who could comment on the case.