Two young men were gunned down on a sunny Monday morning as they sat in a Chevrolet Camaro near a popular San Francisco park, and investigators were on the hunt for two attackers who apparently sneaked up on the victims before opening fire.

The violence near the skate park at Crocker-Amazon Playground marked the city’s 12th and 13th homicides in a year that started off in particularly bloody fashion but saw a sharp downturn in killings over the past month.

The victims were not immediately identified, but police described them as young men, possibly in their 20s, who were sitting in the parking lot in a tan 2002 Camaro around 9:20 a.m. when they were surprised.

Two witnesses at the park, who did not want to be identified, reported hearing eight or nine shots. Police said the gunmen fled on foot, though they did not immediately know a motive for the brazen killing.

Paramedics and police officers found the victims inside the vehicle when they arrived at the park. One man was already dead, while the other was declared dead at San Francisco General Hospital, said Officer Albie Esparza, a police spokesman.

Shell casings littered the parking lot as homicide inspectors interviewed witnesses and collected evidence. Several city gardeners were nearby when the shooting happened and spoke with detectives.

Before the killing, dozens of people were enjoying the park. People walked their dogs on a grassy hillside, while soccer players kicked balls around on turf fields. Afterward, visitors walked along shaded paths, seemingly unaware of the killings.

“It’s a beautiful sunny day,” Esparza said. “It’s a shame this happened. Thankfully, no one else was injured. More people could have been caught in the crossfire. It’s just disturbing.”

Chris Perez went to the park to play soccer and found the crime scene cordoned off with police tape.

“This happens all the time,” said the father of two children, ages 4 and 5, while shaking his head. “I got kids. I don’t want this in my neighborhood.”

The Camaro in which the victims were killed is registered to a woman in Yuba City (Sutter County), but she said she recently sold it and didn’t know the name of the new owner.

While police cleared the parking lot, apparent friends and relatives of the victims showed up to view the scene and broke down in tears.

The killings prompted police, who were worried about possible retaliation, to step up patrols in the area.

San Francisco’s 13 killings this year put the city on a deadlier pace than last year, when there were 45 homicides. However, the city has been relatively peaceful in recent weeks, with no killing since the Feb. 4 shooting death of 37-year-old Kamelei Filivaa on Quint Street in the Bayview neighborhood.

Evan Sernoffsky is a San Francisco Chronicle staff writer. E-mail: esernoffsky@sfchronicle.com Twitter: @EvanSernoffsky