Six people managed to escape the house unharmed due to the actions of one deputy, according to L.A. County Undersheriff Larry Waldie.
A deputy heard the shots fired on the 21900 block of Devlin Avenue at 3:40 a.m., and then saw the suspect, identified as 27-year-old Joseph Mercado, coming out of the house, firing an automatic weapon. According to Waldie, the deputy shot Mercado twice when he pointed the AK-47 at him.
"[The deputy] saw that he had an AK, immediately got his weapon and said, 'Stop, freeze,' and waited until he pointed it at him, and was forced to then fire himself and take the suspect down," said Waldie.
Mercado had allegedly forced his way inside the house by shooting down the door. He had apparently made a statement to police, saying he was going to rectify the dispute.
Family and friends said Mercado had a falling out with his girlfriend. The two recently had a child together.
"I just know that they were seeing each other, they stopped seeing each other, and they were going to court about the child and next thing you know, we're over here on TV hearing this," said Joseph Ruvalacaba, a friend of the victim.
Mercado lived in an apartment building in Montebello. Detectives searched the apartment for evidence Thursday.
Chrissy Cruz says she's known Mercado for six years, and she never expected this.
"He's been quiet," said Cruz. "I don't think he would get into that trouble."
Deputies believe Mercado had plans for even more violence. He allegedly poured a can of gasoline on the side of the house also and lit it. He was carrying a lot of ammunition.
"Personally, I believe that if the deputies weren't here, we would have a lot more people that are deceased in that home, and it probably would have been burned to the ground," said L.A. County Sheriff's Lt. Don Slawson.
Waldie said police found Mercado's ex-girlfriend and her brother murdered upstairs. Several people had jumped off the roof to escape.
Nearby residents said the gunfire sounded like a jackhammer, rattling many from their sleep.
"I had never heard anything like that in real life. It was awful, I didn't know what was going on," said neighbor Mary Canoy.
The ex-girlfriend's mother remains hospitalized in critical condition.
Mercado survived his gunshot wounds and is cooperating with deputies. Authorities said Mercado had a criminal history.