According to LAPD Chief Charlie Beck, the boys were found in the back seat of an SUV. Their father, identified by family as Luis Fuentes, was bleeding from stab wounds in the front seat and taken to Los Angeles County + USC Medical Center in critical condition.
Beck said investigators believe the father of the three young boys is the only suspect in their killings. The motive is being investigated.
Detectives determined that all three boys were killed as a result of multiple stab wounds, Beck said, adding that a knife was recovered at the scene.
It was not immediately clear if the father's stab wounds were self-inflicted.
"These are horrific incidents," Beck said. "These are incidents that scar not only a community but the first responders who have to handle them. It's a sad day in Los Angeles made only slightly better by the fact that we have a probable suspect in custody."
Police received a report of an assault with a deadly weapon around 7:15 a.m. in the 300 block of East 32nd Street, near Dolores Huerta Elementary School.
John Sorrentino, whose furniture store is next to the crime scene, said he was the one who first spotted the bloody scene and called 911.
"I noticed a guy was all bloody as I walked up to the car. As I approached a little bit closer, I noticed a young boy on a passenger side rear seat crouched against the door, blood all over him, and his eyes were half open, and I looked to see if there was any kind of beat on his chest and there was nothing," he said. "I yelled at the guy and he was breathing, but gasping. It didn't look like an attack on him. Doors were closed. He had a box cutter."
Sorrentino said he then saw another boy bent over a seat, and in the back, a leg of the third boy lying upright. That's when he ran inside his store and called 911.
Beck said the deceased children did not attend Dolores Huerta Elementary School, but he did say they were students in the Los Angeles Unified School District system.
Michelle King, the district's deputy superintendent, released the following statement: "On behalf of Superintendent Ramon C. Cortines and all of Los Angeles Unified, I would like to express our deepest sympathy to the family of the three District students who passed away as a result of a non-school related incident. The schools they attended have been notified, and crisis counselors have been provided."
The car the boys were found in was registered to a home in the 4200 block of Woodlawn Avenue. Their mother is deceased, Beck said. A woman believed to be the kids' stepmother was said to be answering questions at the police station.
Luis Chanquin, a relative of the family, laid out candles and cars for each of the boys Wednesday afternoon. He identified the boys as Luigi, Alex and Juan Fuentes.
"They were angels, they were kids, but like I said I just hope God forgives him," Chanquin said. "I know God has a good place for these children up there where he's at."
The suspect remains hospitalized, but is now listed in stable condition. Two prayer vigils for the young victims were held at local parishes Wednesday night.
The family of the three boys has set up a GoFundMe account to help pay for funeral expenses. You can donate here: www.gofundme.com/yk3tdup4.