“I got attitude like they were talking to somebody off the street,” Ms. Rodriguez said. “They wouldn’t even report it,” she added. “They told me to tell her: ‘Don’t go on the block.’”

The feeling of helplessness is spreading among the teenagers.

At a vigil held for the murdered girls before a football game, some students held signs: “Help Us!” “Stop the Violence!” Others shook their heads when Mr. Sini told students to call a hotline for investigative tips.

“We’re the ones out here, dealing with it all,” said a 16-year-old boy who would give only his nickname, Tiny T. “They think they can do something, but they’re just fooling. They can’t do nothing.”

At Kayla’s wake, a 17-year-old student too afraid of MS-13 to give his name said: “You don’t know who’s watching you, who’s following you. Just yesterday, a group of guys in a car with blue bandannas followed a girl home in Brentwood.”

He, his mother and his cousin wore T-shirts that read “Justice for Kayla,” which they had printed at the mall. “Afraid?” his cousin, a 19-year-old woman, said. “There’s not even a limit to afraid.”

At memorials for both Kayla and Nisa, on the cul-de-sac near where their bodies were found, basketballs sat among the glass candles and deflated balloons. Kayla, a tenacious athlete, was going to try out for the varsity basketball team this year. Instead, her mother was starting a scholarship fund called Ball Is My Life.

Ms. Rodriguez hoped her daughter’s death would at least stop the cycle. “It can’t go on anymore,” she said.