Mary Granados, 29, was on the phone with her sister when she was killed in a deadly shooting rampage in Midland and Odessa, Texas, on Saturday.
She is one of seven people who died in the attack, which spanned miles and reportedly started during a traffic stop.
Rosie Granados, Mary's twin sister, told CNN she had been chatting on the phone with Mary when she heard her twin scream.
Mary Granado, a USPS mail carrier, had been nearing the end of her route. The postal service confirmed to USA Today that Granado was an employee.
Rosie Granado told CNN she was confused by the call, and drove along her sister's route.
"I went to help her out. And unfortunately, I did see her laying on the ground," she told the outlet.
"It is the worst pain somebody can go through," Rosie added. "She was just part of my life, part of my whole world. She was taken away from me and my mother, my family."
Officials have yet to identify the victims of the fatal shooting, but said they range in age from 15 to 57. Another 22 people were injured, including a 17-moth-old who is recovering after being shot in the face.
Some of the victims, like Granado, have had their stories shared by friends and family members.
Leilah Hernandez, 15, is the youngest victim of the attack. Her brother, Nathan Hernandez, is hospitalized with injuries. The siblings had been leaving a dealership to pick up a truck Nathan had been saving for when they were shot, their grandmother Nora Levya told The Washington Post.
Joseph Griffith, 40, a former math teacher who was shot waiting at a traffic light with his wife and two children, USA Today reports. His sister, Carla Byrne, told the Post Griffith created close bonds with his students but nothing superseded his devotion to is wife and kids.
Rudolfo "Rudy" Arco, 57, who was shot while driving home from work. His sister Maria Arco told USA Today he had left Las Vegas after the anniversary of the 2017 mass shooting at a music festival in the city, and felt "Odessa was the place to go."
Kameron Brown, an Army veteran who served in Afghanistan, had worked at Standard Safety & Supply for over a year. The company mourned the loss of its team member.
Edwin Pegrino, 25, went outside to investigate the sounds of gunshots when he was hit, The Washington Post reports. He had been visiting his parents in Odessa after moving to San Antonio, his sister told the paper. His husband was shot and is recovering.