A Harlingen mom is mourning the death of her 15-year-old son. 

Kayden Schwartz died on Sunday while riding his bike in San Benito. The Texas Department of Public Safety says someone in a truck struck him on FM 1846; the driver did not stop.

"He had a very bright future, he planned to go to A&M to be a veterinarian," Kayden's mom, Rebecca Schwartz said. "This is unimaginable. I tried to sleep last night, but every time I close my eyes, I can just see him riding his bike."

At 15 years old, her son Kayden was killed while riding his bike. Rebecca shared the last thing she said to him Saturday night.

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"I went into his room and told him I love you, he said, goodnight mom, I love you too," Rebecca said.

She said Kayden snuck out his window early Sunday morning to talk to his ex-girlfriend. When she woke up several hours later to get ready for church, she noticed he was gone.

Rebecca called police to help find him, but her worst fear became a reality when two troopers with the Texas Department of Public Safety knocked on her door.

"It's the most unimaginable thing that anyone could ever tell you, that your son is gone and only 15," Rebecca said.

Kayden was hit and killed at 3 a.m. while riding his bike on FM 1846 near County Road 578. DPS says the driver who hit Kayden from behind did not stop to help him. 

Rebecca hopes her son's last moments were peaceful. 

"I just feel so bad. I wonder what he was thinking, if he was ok," Rebecca said.

Now the family wants whoever took their son's life to be held accountable. 

"I just want whoever did it or if someone knows something to come forward," Rebecca said. "Shame on you for leaving my baby out there to die."

On Tuesday, Harlingen CISD said they have crisis teams supporting students and staff members.

DPS is looking for a truck, possibly a Ford pickup, with chrome trim and damage at the front right side.

Anyone with any information is urged to contact DPS at 956-565-7600.

Watch the video above for the full story.