LOS ANGELES (AP) — An infant girl and her 9-year-old sister who were found on a busy Los Angeles-area freeway were thrown from a moving SUV, and investigators believe their mother was responsible and died by suicide after also killing her partner, authorities said Tuesday.

The 8-month-old was pronounced dead at the scene early Monday, and the older girl was taken to the hospital for moderate injuries, according to law enforcement officials.

The girls’ mother, Danielle Cherakiyah Johnson, fatally stabbed her partner, Jaelen Allen Chaney, in the Woodland Hills neighborhood of the San Fernando Valley, about 25 miles (40 kilometers) northwest of downtown Los Angeles, and fled in a Porsche Cayenne, according to the Los Angeles Police Department.

The couple had an argument in the pre-dawn hours that turned violent, and detectives found a knife “with biological evidence” at the scene, authorities said.

Johnson, 34, then drove along Interstate 405 in Culver City, roughly 10 miles (15 kilometers) west of downtown LA, and her daughters were “expelled” from the SUV while it was moving, the LAPD said. The girls’ names have not been released.

A half-hour later, the LAPD said, Johnson was driving over 100 miles per hour (160 kph) when she crashed into a tree in Redondo Beach, about 20 miles (32 km) southwest of downtown LA. Investigators said the solo crash was an apparent suicide.

The medical examiner identified the 29-year-old Chaney. Although police were called to his apartment after neighbors saw the front door open around 7:30 a.m. and he was pronounced dead there, investigators believe he was killed before the children were discovered on the interstate.