Police responded to the Silver Lake Apartments on East Belmont Avenue in East Central Fresno at about 6:45 a.m. Before officers entered the apartment, they heard a gunshot.
When they entered the resident, they found four people shot to death, including an 18-month-old girl, a 3-year-old boy and a man in his 20s or 30s. A woman in her 20s, believed to be the mother of the children, was found with a self-inflicted gunshot wound.
"This was a terrible tragedy, no matter what the sequence of events, and exactly what happened here," said Capt. Dennis Bridges.
A fifth victim, who had been shot and stabbed, told police that his live-in girlfriend was responsible for the disturbance. He remains in critical condition at a local hospital.
The names of the victims have not been released.
The Associated Press and KFSN-TV contributed to this story.