The man was in critical condition at a hospital, while the boys were pronounced dead and later taken to a coroner's office. A knife was recovered from the front passenger seat, police said.
Detectives believe they have a suspect in custody, but Beck declined to be more specific about that person's identity.
"These are horrific incidents," Beck said. "These are incidents that have scarred not only a community but the first responders that have to handle them."
"It is a sad day in LA," he said.
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.