WATCH: Uvalde Victim Lexi Rubio's Parents Say 'As Her Voice, We Demand Action'

"In the reel that keeps scrolling across my memories, she turns her head and smiles back at us to acknowledge my promise--and then we left."

Kimberly Rubio on Wednesday recounted to members of Congress her last moments with her daughter Alexandria "Lexi" Aniyah Rubio before the 10-year-old was killed last month in a mass shooting at Robb Elementary School in Uvalde, Texas that also left 18 other children and two teachers dead.

Joined by her husband, Felix Rubio, Lexi's mother shared the events of May 24, including a pair of award ceremonies at the school that morning, and a promise to take their daughter--who was recognized with a "good citizen" award and for receiving all A grades--for ice cream that evening. The grieving mother also urged federal lawmakers to pass gun control legislation.

"I left my daughter at that school, and that decision will haunt me for the rest of my life," she told the U.S. House Committee on Oversight and Reform hearing, entitled "The Urgent Need to Address the Gun Violence Epidemic."

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Felix Rubio, a deputy in the Uvalde County Sheriff's Office, had a rare day off, so he headed home after the ceremonies. Kimberly Rubio, a journalist at the Uvalde Leader-News, went to the newspaper's office--where staffers soon began hearing about a shooting on a police scanner.

"It wasn't long before we received word from my son's teacher that they were safe, secure in the classroom," she recalled of 8-year-old Julian Rubio, who also attended Robb Elementary School and was picked up by his grandfather at the local Civic Center after evacuating.

"One of our Robb kids was safe. We focused on finding Lexi. Bus after bus arrived but she wasn't on board," Rubio said. Instead of taking Lexi--one of their six children who attended local public schools this year--for ice cream, the parents spent much of their day searching for their daughter.

"We heard there were children at the local hospital so we drove over to provide her description. She wasn't there," said Rubio, adding that Lexi's grandfather also drove over an hour to check a hospital in San Antonio.

"At this point, some part of me must have realized that she was gone. In the midst of chaos, I had the urge to return to Robb. We didn't have our car at this point, and traffic was everywhere, so I ran--I ran barefoot, with my flimsy sandals in my hand," Rubio recounted, adding that they sat outside until it became clear they would not receive an answer on the scene.

The parents returned to the Civic Center, where they learned their daughter was among the 21 victims. Rubio--through tears--told lawmakers that "we don't want you to think of Lexi as just a number," describing her "intelligent, compassionate, and athletic" daughter who aspired to become a lawyer and "would have made a positive change in this world."

"So today, we stand for Lexi, and as her voice, we demand action. We seek a ban on assault rifles and high-capacity magazines," she said. "We understand that for some reason, to some people--to people with money, to people who fund political campaigns--that guns are more important than children, so at this moment, we ask for progress."

"We seek to raise the age to purchase these weapons from 18 to 21 years of age," Rubio added. "We seek red flag laws, stronger background checks. We also want to repeal gun manufacturers' gun liability immunity."