Covina Police Sgt. Dave Foster said Trujillo's brother called 911 at about 9:15 p.m. Tuesday to report that Trujillo was acting erratically and had a kitchen knife. When authorities arrived at the house on the 100 block of South Waterbury Avenue, Trujillo was outside, but ran back into the residence.
Police surrounded the home, and an officer saw Trujillo inside stabbing himself.
"At that point the officers determined that they needed to get inside as quickly and as safely as they can," said Foster.
Police kicked open the door, and a fight ensued between an officer and Trujillo in one of the bedrooms, according to Foster. Police were able to restrain Trujillo, and then spotted the wounded young boy.
Trujillo and the 2-year-old boy named Nathan were rushed to County-USC Medical Center, where the boy later died of his injuries.
Trujillo had apparently been in a fight with his girlfriend -- the boy's mother -- at the home earlier in the night. The woman had apparently wanted to end their four-year relationship, according to Foster.
"Last night, it had come to a head to the point where she couldn't manage him or sustain the relationship with what was happening to him. And they began to fight about the relationship, where the relationship was going to continue or not," Foster said.
Police said Trujillo lived in the Covina house with his mother. His girlfriend did not live with him; she lived with their son in Baldwin Park.
Neighbors said Waterbury Avenue is a typical, quiet cul-de-sac, and they thought the ambulance they saw was for a medical call.
"We didn't really know anything until early this morning," said neighbor David Bess. "It was gut- wrenching. As a father myself, that was a tough thing to hear."
"Just a quiet neighborhood and just to hear this it's devastating," said neighbor Ray Benavidez. "It's like, your stomach just turns. It's unbelievable, but it's the world we live in."
Trujillo was being held on $1 million bail. He was scheduled to be arraigned on Friday on suspicion of murder.