The reason jurors voted for acquittal on even the least serious homicide charge — involuntary manslaughter — in the Kate Steinle case may have stemmed from San Francisco prosecutors’ decision to raise the stakes in hopes of winning a felony conviction.

Steinle was felled by a ricocheting bullet as she walked with her father on Pier 14 in July 2015. The prosecution charged that the defendant, Jose Ines Garcia Zarate, fired intentionally. The defense argued it was an accident — that he had found a stolen gun on the waterfront that somehow went off.

Jurors were given the option of convicting Garcia Zarate of first-degree murder, second-degree murder or involuntary manslaughter. However, there are two types of involuntary manslaughter — a purely accidental killing, which is a misdemeanor, and one with underlying criminal intent, a felony.

“For example, getting into an accident while texting is doing something that is legal in an illegal way,” said former San Francisco prosecutor Tony Brass. Winning a conviction requires showing that the defendant was guilty of gross negligence, not that he intended to commit a crime.

It’s a similar situation when someone is handling a gun, and it goes off and shoots someone, said Public Defender Jeff Adachi, whose attorneys defended Garcia Zarate.

“The presumption is that, even though it was an accident, you should reasonably know that it could go off and someone could get hurt,” Adachi said. That’s gross negligence.

District Attorney George Gascón’s office, however, opted to give the jury the felony version of involuntary manslaughter, should it decide the Steinle killing wasn’t first- or second-degree murder.

The underlying criminal intent needed for the felony version, prosecutors said, was that Garcia Zarate “brandished” the gun. In his jury instructions, Superior Court Judge Samuel Feng said that meant displaying the gun in a “rude, angry or threatening manner.”

“It would be like someone waving a gun and saying, ‘Get off of my property,’ and the gun goes off,” Adachi said.

No witnesses, however, testified to seeing Garcia Zarate wave or point the gun in a threatening manner at Steinle or anyone else. “A jury would have nothing on which to base the brandishing charge,” Brass said.

District attorney’s office spokesman Alex Bastian said Garcia Zarate pulled the gun out of his pocket, which he said was enough to qualify as brandishing. The defense, however, argued that Garcia Zarate found the gun on the ground — and there were no witnesses to support either version.

“It was one more decision the jury had to make,” Bastian said.

Giant barriers: The Giants are installing 75 planter boxes around AT&T Park — but it’s not for love of urban gardening.

The 4-by-4-foot reinforced concrete boxes — each featuring the team’s logo — are designed to prevent the recent terrorist tactic of driving vehicles at high speeds through crowds.

“We’ve seen what can happen from Nice to New York,” said Alfonso Felder, the Giants’ executive VP of administration.

Besides the 1-ton planters — filled with drought-resistant plants — the team is also installing 60 steel pipes, or bollards, each 4 feet tall, on sidewalks outside the ballpark.

Board save the queen: Sen. Dianne Feinstein was without doubt the highest-profile backer of her former Presidio Terrace neighbors in their fight to get back their private street. But when the Board of Supervisors met to reverse the city’s 2015 tax auction sale of the street to a South Bay couple, there was quite a mix of other supporters in attendance.

They included a religious leader from Temple Emanu-El, down the street from Presidio Terrace, who worried about the devout losing their Saturday service parking privileges on the street.

But maybe the biggest surprise was British Consul General Andrew Whittaker, who lives in a house on the street that his government has owned since 2003. He elevated the dispute to a matter of British national security.

“Security is top concern for the British government when selecting properties overseas, and Presidio Terrace was chosen because it met our security and protocol requirements,” Whittaker said. “Who owns the street and how it is managed has a direct bearing on this. If the sale were allowed to stand, we have significant concerns.”

San Francisco Chronicle columnists Phillip Matier and Andrew Ross appear Sundays, Mondays and Wednesdays. Matier can be seen on the KPIX TV morning and evening news. He can also be heard on KCBS radio Monday through Friday at 7:50 a.m. and 5:50 p.m. Got a tip? Call (415) 777-8815, or email matierandross@sfchronicle.com. Twitter: @matierandross