A teen was using her phone when she stepped into the path of an oncoming semi-truck and was killed, despite her brother's last-minute attempt to save her, San Diego police said.

Fifteen-year-old Nayomi Mendez died in the collision Wednesday in Otay Mesa, an industrial area just north of the U.S.-Mexico border.

Nayomi was a student at Castle Park High School in Chula Vista.

Investigators say the teenager was distracted by her cell phone when she walked into oncoming traffic just after 5 p.m. along the six-lane Otay Mesa Road at La Media Road.

Police say the truck driver was turning right on a green light and saw two pedestrians pushing the crossing light at the corner.

Witnesses told police the girl and her brother were standing on the corner when the girl attempted to cross the street against the light.

“He attempted to grab her, unfortunately she lost her balance and went under the right rear wheels of the trailer,” said Sgt. Art Doherty with the San Diego Police Department.

Nayomi died instantly from trauma to the head.

The truck driver didn’t realize the teen had been hit until he heard a disturbing noise at the back of his vehicle. He stopped the truck and jumped out of the vehicle, officers said.

The girl’s brother was also hit by the truck and suffered a shoulder injury.

The brother was traumatized emotionally, Sgt. Doherty said.

Several members of the teen’s family arrived at the scene. San Diego police will provide counseling, Doherty said, but added that it was a difficult scene for investigators.

“Good family. It’s just a sad, sad situation,” Sgt. Doherty said.

“You just feel for the members. It’s hard to comfort them,” he said.

Castle Park student Alejandro Jimenez knew Nayomi for more than three years. He and other students met with grief counselors after hearing the tragic news Thursday morning.

"I was just sad. I couldn't believe it," Alejandro said.

"I can't concentrate. I can't do my work."

Investigators do not believe alcohol was involved in the accident. The truck driver will not be charged.

Another driver, John McKenzie, was coming home from work when he witnessed the crash. He said he has seen several accidents at the busy intersection that’s near several auto and trucking companies, a 7-Eleven and other restaurants.

“This isn’t the first time. It’s just, people need to pay attention and stay on the sidewalk. I’ve even seen trucks cut up on the curbs,” McKenzie said.

Officers were not sure why the brother and sister were heading east on Otay Mesa Road. They were told the two had been picked up from a nearby high school and dropped off near the intersection.

Several Castle Park students tweeted asking other students to wear black on Friday to honor Nayomi.