Kristin Hermeler, 29, who committed suicide Monday, called herself "someone who has been rejected, victimized and ostracized" in a letter to Columbine survivor Brooks Brown months after the April 20, 1999, high school slaughter, the Denver Post, reported citing KCNC-TV in Denver.
"I just wanted to write to you and let you know that not a day goes by that I do not think about what happened," she wrote.
Her sister, Candice, survived the .22-caliber bullet she fired into her forehead at the same moment her sister killed herself.
"It was a very sweet letter, very sad," Brown told the Post Saturday. "It was her wanting to know why it happened, trying to understand."
Brown said they had not been in contact for years and the twins did not tell him they were visiting Colorado.
He said Kristin told him she and her sister had been bullied "physically and mentally."
The suicide pact remains a mystery, and Candice has refused to talk. She remained in serious condition Saturday night. Her parents arrived in Denver Friday.
In the Columbine massacre, two students killed 12 students and a teacher before killing themselves.