In the chaos of bullets and bombs and wailing fire alarms, Erin hid for hours behind an upturned table.
She says, "It was scary, but the sound of the sirens didn't really bother me because it helped me know that I was still alive.
She was in the science room where students tried in vain to save the life of teacher Dave Sanders.
"They asked for all the shirts and sweaters and jackets that anyone could give up, to help him stop bleeding," says Erin.
Outside, Erin's mother Jackie was among the anxious, waiting parents. She says, "I was lucky that day. I walked away with my kids. I walked away."
But the year that has passed has been a test for Erin, just as it has been for all of Littleton.
In December, the community's nerves were rattled when Erin received an email warning there would be another slaughter at Columbine.
Erin explained the email's message, "He told me not to go because he doesn't want my blood on his hands."
Columbine closed down while police investigated. It turned out to be a hoax by a Florida teenager. But in February, there was real life horror when two Columbine students were murdered in a sandwich shop. One of the victims was Erin's friend and next door neighbor.
"How do I tell Erin it's okay? I don't. I can't. I can't," says Erin's mom.
For Erin Walton there was to be one more encounter with violent death this traumatic year. Two months ago she was in a parking lot just down the street from Columbine when an armed man being chased by the police pulled in almost beside her and shot himself to death.
She says she saw his body being pulled out of a Jeep. "I just started, break down crying again," says Erin.
"That sort of falls into kind of what we call too much, and too ugly, and too long," says psychologist John Nicoletti.
Nicoletti and Lotte Flater have been treating survivors of Columbine during a year that has left many there with little chance to heal.
"When you've been through a trauma, you're a little bit more vulnerable to the next trauma. Even if you've dealt with it, there's still kind of the scar tissue that's developed there," says Flater.
The wounds in Erin's family go deep. Her mother says she and Erin are no longer as close. "I've seen the ugly side of Erin and she's seen the ugly side of me. In the senses of... we both battle our own demons about that day."
One point of contention is that Jackie Walton pulled her daughter out of Columbine after the email incident and put her in a school across town. Erin wants to go back.
"The everyday surroundings there were hard to handle, but I miss my friends and my teachers," explains Erin.
"I know Erin wants to go back to Columbine and I'm ready to accept that. I'm just afrid for that day, actually," says Erin's mother.
Erin also is a little uncertain -- but she's hoping her horrible year and Littleton's horrible year is over.
"I'll always want to remember what happened," says Erin. "I survived this so basically I think I could survive anything."