Maria Munoz and her two sons were struck last November as they stood on the sidewalk.
Deputy Carrie Robles-Placencia was training at the time of the crash.
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The DA's office says there is insufficient evidence to prove that Robles-Placencia committed vehicular manslaughter.
According to investigators' reconstruction of the incident back in August: the sheriff's patrol vehicle was headed southbound on Indiana Street when it stopped at a red light at Whittier Boulevard. That is when the deputies responded to a call of an assault with a deadly weapon involving a gunshot victim.
The deputy behind the wheel was still in training and her trainer was in the SUV with her.
The SUV turned on the flashing lights, but not the siren.
The patrol car then accelerated into the intersection against a red light and struck a Honda Accord heading east on Whittier. That car spun out of control and hit a Honda Odyssey stopped at the light.
The sheriff's SUV swerved to the southeast, hit a pregnant pedestrian and then drove up on the sidewalk along Whittier. The SUV bounced off the side of a Wells Fargo bank, then struck the mother and her two children.
It continued out of control, struck another pedestrian and a cement trash can and then one more pedestrian.
The two boys, identified by family members as Marcos and Jose Luis Hernandez, died and seven others were injured.
The witness who saw the crash told Eyewitness News that a Metro bus was blocking the view of the Honda Accord as it entered the intersection on the green light. The Honda driver couldn't see the flashing lights of the patrol vehicle and there were no sirens as an audible warning.
That lack of a siren was a key element in the DA's investigation of a possible manslaughter or felony reckless driving case.