On Tuesday, the sixth day of the Kate Steinle murder trial, a San Francisco Police Department criminalist testified that the gun involved in Steinle’s death could not have fired as a result of being hit or dropped. But the defense’s main argument is that the shooting two years ago on Pier 14 was an accident.

“My opinion is that the trigger has to be pulled for the gun to be fired,” Gerald Andrew Smith told prosecutor Diana Garcia from the stand.

Smith is the examiner who identified the gun — a SIG Sauer P239 .40 caliber semi-automatic pistol — as the weapon that fired the bullet that killed Steinle. A police diver retrieved in from the bottom of the bay and delivered it the SFPD laboratory, where Smith works in a lockbox full of murky seawater.

Garcia questioned Smith for nearly an hour and a half Tuesday, asking him to describe in detail the various internal safety features of the gun, most notably the firing pin block, a device that prevents the firing pin from moving unless the trigger is pulled.

She also asked Smith to explain the “trigger pull” tests he conducted during his examinations. In those tests, he fired the gun several times using a hook attached to a weight meter to determine how many pounds of pressure were required to fully depress the trigger.

He testified that, depending on which mode the gun was in — single-action or double-action — between 4.5 and 9 pounds of pressure was needed.

According to Smith, neither amount of pressure could be quantified as “light.”

Garcia presented as evidence both the gun, with a zip-tie looped through its barrel for safety, and the bullet, flattened and grooved from where it ricocheted off of the pier before it hit Steinle in the back.

As close-up images of both items were projected over the courtroom, the defendant, Jose Ines Garcia Zarate, sat staring at the screen. The small-framed man wore a large set of headphones, through which Spanish interpreters translated the proceedings. 

Speaking outside of the courtroom during a break, Garcia Zarate’s attorney, public defender Matt Gonzalez, said he didn’t believe that the presence of internal safety mechanisms was enough to rule out the possibility that the gun was fired accidentally.

“Basically what they’re calling the safety device on this gun never keeps you from being able to pull the trigger,” he said. “The only thing that it guarantees is that in order for the gun to fire a projectile, the trigger has to be activated. Now that can be with a finger, it can be with an object that gets caught into it, it can happen on accident, it can happen on purpose.”

Back in court under cross-examination, Gonzalez asked Smith to describe each of the gun’s safety features in detail, and had him confirm that the model has no external feature that would prevent the trigger from being activated.

After several minutes of back and forth, Smith said that the gun’s safety features prevent firing only “in the absence of the trigger being pulled.”

“I like the way you said it that time,” Gonzalez said. “Let’s move on.”

He then asked Smith if he was aware that between 2005 and 2011, the SFPD reported 29 accidental discharges of their standard weapon, the SIG Sauer P226, a slightly larger but very similar model of pistol to the one used in the shooting.

Gonzalez also questioned Smith about the weight it takes to pull the gun’s trigger. Citing instances of toddlers accidentally firing guns, he asked Smith to clarify that the weights don’t signify that pulling the trigger is the same as lifting a bag of the same weight with your finger, but rather akin to nudging a book of that weight across a countertop.

Gonzalez’s questions seemed to be establishing the groundwork for his argument that Garcia Zarate found the gun that killed Steinle wrapped in a T-shirt while searching for cans and bottles near the pier, and that it accidentally discharged as he was handling it.

The centerpiece of this argument is the fact that the bullet first hit the pier, then ricocheted and struck Steinle in the back. Gonzalez has said in press conferences that this evidence, presented in court on Monday, indicates that his client did not aim the gun at the victim.

Garcia Zarate is an undocumented Mexican immigrant, who has been deported from the United States five times in the past. He had been released from jail just before the shooting, a fact that was not disclosed to Immigration and Customs Enforcement because of San Francisco’s status as a sanctuary city.

President Donald Trump referenced the shooting at the Republican National Convention last year, placing it at the center of a national debate about immigration law and the rights of sanctuary cities.

The trial, which began on Monday, Oct. 23, is expected to continue for at least a month.