Investigators arrested Martin Martinez Sunday after discovering the bodies of his 6-month-old daughter, his mother, a woman he'd been in a relationship with and two other children inside a home in Modesto, California.
Police identified the two adult victims: Amanda Crews, a physician with whom Martinez had a relationship; and Anna Brown Romero, Martinez's mother.
They haven't detailed what evidence they have allegedly tying the 30-year-old suspect to the weekend deaths, but a police spokeswoman described said he'd been booked in jail "on suspicion of murder."
Just last week, a doctor determined that the October death of a 2-year-old boy in Martinez's care was a homicide, not an accident, police announced Monday, describing Martinez as a suspect in that case. The mother of that boy was Amanda Crews -- one of the people killed over the weekend.
Authorities aren't yet releasing a motive or detailing the victims' cause of death.
A family member who asked not to be identified told CNN that two of Crews' daughters -- 6-year-old Elizabeth Ripley and 6-month-old Rachael -- were among the dead. Rachael, whose last name was not immediately known, was also the daughter of the suspect, Martinez, the family member said.
Police said a 5-year-old girl, a distant relative who'd been dropped off to play at the house that morning, was also killed.
Investigators discovered the victims' bodies in the Modesto home Saturday after Crews' friends called and asked them to check on the house when she didn't show up for a lunch date, Modesto Police Chief Galen Carroll said. Hours later, they named Martinez as a "person of interest" in their homicide investigation, posting his photo on social media and asking anyone with information on his whereabouts to call 911.
Around 1 a.m. Sunday, police in San Jose, California, arrested Martinez in a shopping mall parking lot after he left a movie with his father, Carroll said.
Martinez is being held in the Santa Clara County Jail without bail, according to jail records. It was not immediately clear whether he had legal representation.
Investigators are still searching for witnesses to build their case, Modesto Police spokeswoman Heather Graves said.
Police chief defends response
The 2-year-old boy who died last year, Christopher Ripley, was also Crews' child.
A neuropathologist gave police "verbal notification" Thursday that the 2-year-old died "as a result of a homicide," Carroll said. But the coroner's report in the case has not yet been completed, the police chief said.
Because of the circumstances of the Modesto killings, Graves said, investigators "sped up the 2014 case for the safety of the community and because (Martinez) is a flight risk." But normally, she said, police wouldn't charge someone before the coroner's final report is completed.
Asked whether the weekend deaths could have been prevented if authorities had worked more quickly to solve the October case, the police chief defended his department's response, noting that it takes time and many agencies to conduct a homicide investigation.
"The police department did not drop the ball. It is horrible that this tragedy happened," he said, "but there were no indications leading up to that time that anything like this would occur."
Christopher Ripley's obituary
last year said the 2-year-old "passed away suddenly, leaving broken hearts everywhere."
"He loved playing with his building blocks, his toy trains, and loved reading his favorite books 'Good Night Moon' and 'Hungry Caterpillar,' " the obituary said. "One of his favorite songs was 'Twinkle Twinkle Little Star.' "