When prosecutors filed felony charges Tuesday against a San Francisco city gardener who ran over a woman as she lay with her baby in a Bernal Heights park, they concluded that his decision to steer a pickup truck down a grassy hill represented a reckless crime rather than a tragic accident.

The charges against 58-year-old Thomas Burnoski - vehicular manslaughter and hit-and-run - mark the latest chapter in a painful case that raised questions about whether Recreation and Park Department workers followed city policies designed to protect park visitors.

Prosecutors have been studying the case since Sept. 5, when police say Burnoski struck and killed Christine Svanemyr, 35, at Holly Park, before driving away. Prosecutors believe he purposefully fled after a grossly negligent act, but Burnoski's attorneys said Tuesday that the explanation was simpler: He made a mistake and didn't initially realize it.

$350,000 bail

Burnoski, a San Francisco resident, turned himself in Tuesday afternoon to face the charges leveled by the office of District Attorney George Gascón. Burnoski was jailed in lieu of $350,000 bail and is scheduled to be arraigned Wednesday.

Burnoski was arrested by police on the day of the incident, but he had been free on $25,000 bail during the investigation. Members of Gascón's office did not explain the factors that went into Tuesday's decision, but the charges suggest they believe Burnoski acted recklessly.

The case is certain to turn on Burnoski's mind-set before and after he ran over Svanemyr, a San Francisco resident, in a city-issued truck as the woman relaxed in the grass with her dog and her then-10-month-old baby.

Burnoski's attorneys said he veered onto the lawn from a paved pedestrian path to avoid an unleashed dog. However, park department policy does not allow workers to drive on pedestrian paths.

After hitting Svanemyr, the vehicle continued down the grassy hill, hopped off a curb to Holly Park Circle, which surrounds the park, and drove to a staff meeting at nearby St. Mary's Park. According to his attorneys, Burnoski then told his supervisor that he believed he hit something, but he didn't know what, and they returned to Holly Park.

"We believe in this case it was a tragic accident, but it wasn't criminal," attorney Tony Tamburello, whose firm is representing Burnoski, said Tuesday.

"From everything we know, the facts do not support the type of charge which requires recklessness and gross negligence," Tamburello said. "With respect to the hit-and-run in particular, he was on his way back to the scene. He was not running from anybody. He was doing exactly what he was supposed to do."

Prosecutors said earlier that they were waiting for the results of Burnoski's toxicology report before filing charges. Those results were not released Tuesday.

The park department's vehicle policy bars workers from driving on pathways, sidewalks, closed roads or the actual park area "merely for convenience purposes." If someone must drive in those areas, they are required to have a second staff member act as a spotter.

No spotter was in place Sept. 5, according to police reports, and the pedestrian path that Burnoski was said to have veered away from was a significant distance from the location where Svanemyr was struck.

Neighbors who frequented Holly Park, as well as other San Francisco residents, complained after Svanemyr's death that park workers often flouted the department's vehicle policy.

New speed-limit policy

At a Board of Supervisors hearing held in response to the accident, the park department's general manager, Phil Ginsburg, said there had only been four incidents in the past 20 years involving pedestrians and department vehicles.

Svanemyr's death was the first vehicle fatality in the department's history.

Ginsburg said the department is working to improve signs on vehicle service roads that also serve as pedestrian paths, and is implementing a 5-mph speed limit in parks. The department did not have a speed-limit policy in place at the time of Svanemyr's death.

Burnoski, a parks employee since 2006, was placed on unpaid leave after Svanemyr's death. He had no recent criminal record, though he was cited last year for talking on a cell phone while driving, according to the California Department of Motor Vehicles.

Tamburello said Burnoski was devastated by the incident. The gardener's 20-year-old daughter, Tiffany Burnoski, had died in a car accident in April in Fresno County.

"He understands what this all means and is devastated and feels very strongly," Tamburello said. "He relates to the child that has to grow up without a mother and a husband who has to parent by himself. He understands all that. It's been very difficult."

Svanemyr's family did not return calls for comment Tuesday.