The bullet that killed a woman on a San Francisco pier two years ago ricocheted off the ground about 100 yards away before hitting her in the back and later launching a criminal case at the center of a national immigration debate.

A police officer who helped supervise the investigation into the death of Kate Steinle testified about the bullet's trajectory Monday at the trial of the Mexican national charged with murder.

Officer John Evans, who retired last year, said investigators found a mark on the pier's concrete floor four days after the July 1, 2105 shooting. Lawyers for Jose Ines Garcia Zarate argue that the ricochet shows the shooting was accidental.

Prosecutors charged Garcia Zarate with murder because they say he meant to point the gun at a crowd of people and pulled the trigger.