Martin Martinez allegedly killed a woman he once had a relationship with, his own daughter and three other people, Modesto Police Department spokeswoman Heather Graves said Sunday.
Authorities aren't yet releasing a motive or detailing the victims' cause of death, she said, but they've determined that all five of the victims were family members.
The three slain children ranged in age from 6 months to 6 years, Graves said.
"It's devastating, not only to the families of the victims and to the neighbors here as well, (but also) to our law enforcement community," she said, noting that a chaplain had been at the scene of the killings providing counseling.
Investigators are still searching for witnesses to build their case against Martinez, Graves said.
"He's been booked into the Santa Clara County Jail on suspicion of murder," Graves said. He is being held without bail, according to jail records. It was not immediately clear whether he had legal representation.
CNN's Patrick Cornell, Joe Sutton and Vivan Kuo contributed to this report.
By Catherine E. Shoichet