MODESTO, CALIFORNIA: A man suspected in the homicides of five people — including his mother and baby daughter — had also been linked to the death last year of a toddler, police said.

Martin "Marty" Martinez, 30, was arrested Sunday in the deaths of the five, whose bodies were found a day earlier.

Modesto Police Chief Galen Carroll said at a news conference Monday that a pathologist concluded last week that 2-year-old Christopher Ripley died of "blunt force trauma" to the head on Oct. 2, 2014, two days after he was taken to a hospital.

Carroll said Martinez had been under investigation since the boy's death and that authorities were preparing to formally charge him with homicide when the five bodies were found Saturday afternoon in the Modesto home he used to share with Dr. Amanda Crews.

Crews, 38, was Ripley's mother and one of the five homicide victims. Martinez's mother, his daughter with Crews and two other girls were also found dead. Martinez is a suspect in the five homicides, Carroll said.

Carroll said police obtained a warrant for Martinez's arrest for the toddler's death Saturday night, after the five bodies were discovered.

Carroll said police didn't issue a warrant earlier because they were awaiting the pathologist's written report.

"The Modesto Police did not drop the ball," Carroll said. He said the investigation of Ripley's death took nine months because the department had to hire an outside pathologist who specialises in neurology to help with the case.

"Homicides do take a great deal of time to investigate," Carroll said. Carroll said a "limited" number of law enforcement officials knew of the pathologist's verbal report delivered to police on Thursday that Ripley was a homicide victim. Carroll said he believes Martinez didn't know of the pathologist's report.

The arrest warrant issued for Martinez in the toddler's death shows the pathologist found the "method of death was consistent with Christopher's head hitting the tile floor as a result of abuse," the Modesto Bee reported.

Carroll said Stanislaus County Child Protective Services had ordered Martinez to stay away from Crews' surviving children.

"We do not believe that played a factor in this incident," Carroll said of possible motives for the five homicides. Their bodies were found at about 3:30 p.m. Saturday after police received a call asking for a "security check" of the home.

The daycare Christopher attended reported he suffered three suspicious injuries around the time the boy began potty training. Crews denied Martinez caused the injuries but acknowledged that he was present during two of them, according to the arrest warrant.

Carroll declined to divulge the cause of the five deaths. He said investigators haven't yet determined a motive.

Crews was a doctor and worked for the Stanislaus County Health Service Agency, according to the California Medical Board. The agency didn't return several phone calls.

Modesto Police spokeswoman Heather Graves said counselors and chaplains are available for the officers who first entered the home and made the grisly discovery. A group of law enforcement officials could be seen huddled together and praying in front of the house shortly after the discovery of the bodies.

The house is in an upscale subdivision lined with four- and five-bedroom homes that were built less than 10 years ago.