Story highlights Parents of Claire Davis announce her death "with unspeakable sadness"

High school senior was shot December 13 by a classmate, who then killed himself

Karl Pierson had been seeking revenge against a librarian, police say

Claire Davis, the 17-year-old high school senior shot by a classmate at their Colorado school, died Saturday at the hospital where she was being treated.

"It is with unspeakable sadness that we write and say that Claire has passed away from the gunshot wound she received at Arapahoe High School on December 13," her family said in a statement posted on the Facebook account of Littleton Adventist Hospital.

"Although we have lost our precious daughter, we will always be grateful for the indelible journey she took us on over the last 17 years -- we were truly blessed to be Claire's parents. The grace, laughter and light she brought to this world will not be extinguished by her death; to the contrary, it will only get stronger."

Davis was killed after Karl Pierson, 18, according to police, walked into Arapahoe High School armed with a pump-action shotgun, a machete and a backpack containing three Molotov cocktails, a bandolier of ammunition across his chest.

He shot Davis once in the head and fired randomly into school hallways. Police said Pierson was looking for revenge against a librarian, but couldn't find the faculty member and killed himself as a school resource officer closed in.

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Davis did not know Pierson and was just a random victim, police surmised. She had been in a coma since the shooting.

Davis was described as a sweet girl who loved horses and the English boy band One Direction.

After friends started a Twitter campaign, the band recorded a 13-second video message and sent it directly to the Davis family, a source close to One Direction told CNN on Thursday. The video shows singer Liam Payne delivering the message as the other four members stand by.

The hospital said Davis died at 4:29 p.m.

The family's message on Facebook thanked the hospital staff, first responders and law enforcement officers.

"Last week was truly a paradox in that we lost our daughter, yet we witnessed the wonderful love that exists in the world through the tremendous outpouring of support we received," the family said.

Pierson's parents issued a statement last week.

"We cannot begin to understand why Karl did what he did," Barbara and Mark Pierson said. They also offered prayers "for the entire Arapahoe High School community, as we know your lives are forever changed by this horrific event."

Hundreds of people attended a vigil for Davis last Saturday.

The shooting happened about 10 miles from where Eric Harris and Dylan Klebold killed 12 students and one teacher in a rampage at Columbine High School in April 1999 before killing themselves.