SAN FRANCISCO >> The bullet that killed a San Francisco woman whose death touched off a national debate about illegal immigration ricocheted off the ground about 100 feet away before hitting her in the back, a retired police investigator testified today.

Former Officer John Evans said he and other investigators working on the case found a “strike mark” on the pier’s concrete surface four days after the shooting of Kate Steinle by a Mexican national who had been deported five times.

Investigators had overlooked the mark on the night the 32-year-old Steinle was killed, said Evans, who later retired from the department.

Authorities returned to the popular pier four days later, after the bullet was found to be partially flattened, indicating it had ricocheted, he said.

Lawyers for defendant Jose Ines Garcia Zarate say the ricochet shows the shooting was accidental.

Prosecutors have charged Garcia Zarate with murder, alleging he intended to point and shoot the gun at pedestrians on the pier on July 1, 2015.

Evans also testified that inexperienced shooters are often shaky and fire in haste, causing the barrel of the gun to point downward. He called it “jerking the trigger.”

Defense lawyer Matt Gonzalez called that aspect of Evans’ analysis “highly speculative,” and the two wrangled over whether the shot had traveled straight, which would support the prosecution’s contention that Garcia Zarate aimed the gun before firing.

Evans conceded on cross-examination that he didn’t know exactly where Garcia Zarate was sitting when the gun fired nor did he know the specific spot where Steinle was standing when she was struck.

The retired officer said he drew a circle around blood spots and Steinle’s bloody clothes and another circle where witnesses placed Garcia Zarate. Using a laser pointer, Evans said he created a straight line between the two circles and through the strike mark.

Prosecutors and Gonzalez said the case boils down to whether Garcia Zarate pointed and fired the gun intentionally or the weapon accidentally discharged.

The shooting sparked a political furor during last year’s presidential race, with then-candidate Donald Trump citing the killing as a reason to toughen U.S. immigration policies.

Garcia Zarate had been released from the San Francisco jail about three months before the shooting, despite a request by federal immigration authorities to detain him for further deportation proceedings.

San Francisco is a sanctuary city, with local law enforcement officials barred from cooperating with federal immigration authorities. President Trump has threatened to withhold federal funding to cities with similar immigration policies.

Garcia Zarate was arrested shortly after Steinle died in the arms of her father, who has attended nearly every day of the trial with his wife and son.

Garcia Zarate said he found the gun wrapped in a T-shirt on the pier and it accidentally fired when he picked it up.

A Bureau of Land Management ranger reported the semiautomatic handgun had been stolen from his SUV several days before the shooting. A police diver found it in San Francisco Bay the day after the shooting.