SAN FRANCISCO » A California city has moved to install surveillance cameras along a stretch of highway as authorities investigate a spate of freeway shootings that have taken the lives of six people, including a young mother of four, in the suburbs east of San Francisco.

The decision on Monday by Pittsburg is the latest action aimed at solving and stopping the attacks that also injured 11 people in the past year.

Police say all 20 drive-by shootings on or near two major highways have been gang-related.

“Without exception, these cases … are not random,” California Highway Patrol officer Daniel Hill said.

Freeway shootings have occurred for decades throughout California. But the frequency and scattered locations of the latest attacks have frightened motorists and snarled traffic.

The California Highway Patrol, FBI and two dozen local law enforcement agencies have launched a task force to stop the violence, but so far just one person has been arrested on suspicion of wounding a motorist on April 1.

The latest shooting occurred on May 10, when 25-year-old Shanique Maria was shot and killed on Highway 4 in Pittsburg.

Authorities say a shooter riding in a white Mercedes gunned down Maria while she was a passenger in a black Lexus on road linking San Francisco and Stockton.

The Lexus veered off the highway, hit a dirt embankment and rolled over as the shooter sped away.

Other shootings occurred along a six-mile stretch of Interstate 80 near Richmond and on nearby connector highways and freeways.

The Pittsburg City Council agreed to spend $100,000 to install the surveillance cameras along the four miles of Highway 4 that cut through the blue-collar city of 65,000 people about 40 miles east of San Francisco.

“You have people that are concerned,” Councilman Sal Evola said.

Police in the city already have access to more than 200 cameras at major intersections and shopping malls. The new cameras will be plugged into the city’s existing surveillance system.

Evola said city leaders began considering the highway cameras after 28-year-old Uriel Moreno was shot to death in April. It was the city’s first homicide of the year.

The cameras took on a new urgency with the killing of Maria, 25, whose children range from 9 months to 5 years old.

Police say it appears that victims were initially targeted on surface streets then followed onto highways and shot. The shootings have occurred at all hours of the day.

“It’s an easy getaway and no one hears the gunshots,” Pittsburg police Capt. Ron Raman said. “Witnesses called our dispatch with reports of a collision” after the killings of Maria and Moreno.

The task force is conducting searches of suspects on parole or probation and is obtaining warrants to search homes. Officers have arrested 77 people on suspicion of gang activity.

California Highway Patrol officer Daniel Hill said a number of surviving victims have not been cooperative. Authorities are seeking help from any witnesses, even if they think their information is insignificant.

“These cases could turn on the smallest detail,” Hill said.