"We conclude there is some rational ground for assuming the possibility that Costa was aware of the risk to human life posed by continuing to drive his large semi-trailer down the highway, and that he consciously and deliberately disregarded that risk," the court wrote.
The trucker from Everett, Mass., was hauling cars over the San Gabriel Mountains on April 1, 2009, when his brakes failed on the steep Angeles Crest Highway above La Canada Flintridge.
The double-decker rig hit a car, killing 58-year-old Angel Jorge Posca and his 12-year-old daughter Angelina, of Palmdale, collided with four other vehicles and smashed into a bookstore at Angeles Crest Highway and Foothill Boulevard, injuring three others.
Aside from the murder charges, Costa faces two counts of vehicular manslaughter and three counts of reckless driving causing injury.
In October, Lench threw out the murder charges, saying there was insufficient evidence to show Costa intended to kill the victims.
L.A. County prosecutors appealed, arguing that the lower court had "acted in excess of its jurisdiction contrary to statute" and "abused its discretion" in dismissing the murder charges.
Costa, who had a class A commercial driver's license, was required by law to inspect his vehicle, which the D.A. maintains he didn't. Prosecutors said Costa ignored a warning sign that said rigs the size and weight of the one he was driving are barred from that stretch of winding highway.
"We allege he did not do that inspection because if he had he would have known that five of his ten brakes were not working," said Jane Robison, a spokeswoman for the L.A. County District Attorney's Office. "In addition, he had witnesses who pulled him over and told him his brakes were smoking. One said that you can't proceed down that hill, it's going to be too dangerous."
An off-duty firefighter noticed the truck's overheated brakes giving off smoke. He asked Costa to pull over and advised him that the highway was a bad route for semi-trailer trucks and that he should either turn around or take an alternate route and drew a map for him, according to a grand jury transcript cited by prosecutors in their appellate court request.
"We're pleased with the decision and we're moving forward with the trial," Robison said shortly after the appellate court panel's decision.
Defense attorney Steve Meister said the case doesn't merit murder charges.
"I am disappointed. I think the opinion is unsound legally, intellectually, and morally. I think the people who are supposed to be responsible in the Justice system are thirsting for my client's blood," said Meister.
Costa is due back in Pasadena Superior Court on April 30 for a status conference, where a judge will likely reinstate those two charges of second-degree murder. If he is convicted on the murder counts, he could face 30 years to life in prison.
After the accident, La Canada Flintridge officials accused the California Department of Transportation of failing to respond to safety concerns about trucks on the road and previous accidents at that intersection.
A couple of days later, Caltrans instituted a 90-day ban on five-axle trucks along Angeles Crest Highway between the Angeles Forest Highway and the 210 Freeway. The ban was later made permanent.
The Associated Press and City News Service contributed to this report.