Claire Esther Davis, a 17-year-old senior at the high school, was shot in the head by Pierson shortly before killing himself, Sheriff Grayson Robinson reported on Saturday.
Robinson also acknowledged that previous official reports incorrectly listed Davis as a 15-year-old female student.
"Our beautiful daughter Claire Davis has severe head trauma as the result of a gunshot wound,” the family said in a statement. “She needs your continued prayers.”
Sheriff Robinson also said that Pierson entered the school intending to hurt many people and that the incident took 1 minute and 20 seconds, from the time he entered the school to the time Pierson took his own life.
"The gunman [Pierson] entered the school with a pump shotgun with multiple rounds and the intention to use them to hurt people," Robinson said on Saturday.
Robinson said video surveillance showed Pierson carrying a pump-action shotgun, a bandolier full of ammunition and a backpack that was later discovered to include three incendiary devices resembling a Molotov cocktail.
Law enforcement officials late on Friday night identified the student gunman as 18-year-old Karl Halverson Pierson.
“Our initial investigation is causing us to believe that this shooting is the result of revenge on the part of the shooter, because of a confrontation or a disagreement between the shooter and the teacher,” Arapahoe County Sheriff Grayson Robinson said during an afternoon press conference.
The teacher, who officials are not naming at this time, got wind that a gunman was looking for him and left campus immediately. Sheriff Robinson said that action by the teacher probably saved lives.
“He knew he was the target and he left that school in an effort to try to encourage the shooter to also leave the school with the focus of safety and security and well-being of our students in his mind,” Robinson said.
Regardless, one student was shot and it wasn’t clear last night if the gunman targeted her.
A short time after the initial shots, Sheriff Robinson said deputies found the assailant in a classroom with a self-inflicted gunshot wound.
Robinson praised the fast response times, noting officers trained for a scenario like this recently.
“Those protocols, unfortunately, have had to be used far, far, far too many times in Colorado and across the United States because of these tragic episodes where people decide they have got to even up the score with a weapon,” Sheriff Robinson said.
The shooter was also armed with two Molotov cocktails, one of which was detonated during the attack, causing a small fire.
Deputies on the scene reported encountering a cloud of smoke in the school but there were no reports of injuries from the fire.
At an evacuation point across the street from the high school, teary-eyed parents hugged sons and daughters.
Arapahoe High sophomore Sofia May said there was confusion after the first gunshots rattled her class.
“At first I thought maybe someone dropped a book or something like that,” May said. “People were looking around and I realized it was gun shots -- we thought it was a drill at first, but obviously not.”
Her mom, Denise Andert, looked extremely relieved as she reunited with her daughter.
Andert echoed many parents and kids here in her disbelief that this could happen to them.
“You always see it on TV,” Andert said. “You always like assume it’s going to happen to everybody else, or just not happen again. It’s insane.”
Andert and her daughter live in this neighborhood by the school, which is less than 10 miles from Columbine High School, the scene of a horrific mass shooting 14 years ago.
The significance of that wasn’t lost on Governor John Hickenlooper.
“To have this all too familiar sequence where you have gunshots, and parents racing to the school, and unspeakable horror in a place of learning,” Hickenlooper said.
Saturday the 14th is also the one year anniversary of the mass shooting at Sandy Hook Elementary in Newtown, Conn., although Sheriff Robinson says he believes that’s a coincidence.
Jay Keller, Megan Verlee and Ben Marcus contributed to this report.