One of the children, an eight-month-old baby girl, was killed while her nine-year-old sister was injured after they were "expelled" from a Porsche SUV on Interstate 405 in the early hours of Monday morning.

Police had been investigating the trail of death and violence to see if the incidents were connected before finding the children's mother Danielle Johnson to be responsible.

The 34-year-old had gotten into an argument with Jaelen Chaney, her boyfriend who she lived with, and stabbed him with a knife at around 3:40am local time (11:40am UK time), police said.

Johnson then took her children out in her car before "the two children were expelled from the vehicle while it was moving" on Interstate 405, the force added.

Investigators believe the children fell or were thrown out of the moving vehicle, the California Highway Patrol said.

Police have said the nine-year-old girl was taken to hospital with moderate injuries.

Johnson then sped into a tree in Redondo Beach, a coastal city in the Los Angeles region, at more than 100mph, police said.

The fatal collision occurred at around 5am local time (1pm UK time).

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Investigators later found Mr Chaney, 29, dead in the Woodland Hills home where he had lived with Johnson and her children, police said.

The property is around 30 miles north of Redondo Beach, where Johnson's body was found after the crash.

"We really don't know why this incident escalated to such violence," Police Lt. Guy Golan said, according to Sky News' US partner network NBC Los Angeles.

The highway patrol said it received reports of a medical emergency at 4:29am local time (12:29pm UK time) about the injured children on the freeway.

The baby was found to have serious injuries and was pronounced dead by the Culver City Fire Department at 4:44am (12:44pm), the highway patrol said.

The 405 Freeway is the main artery linking the western part of the valley to the Los Angeles basin.

The surviving child is in the care of Child Protective Services, NBC Los Angeles reported.