Colorado school shooting

Students comfort each other at Arapahoe High School in Centennial, Colo., after a student opened fire. (Ed Andrieski / Associated Press / December 13, 2013)

Shock,  horror and grief poured out over social media Friday as an all-too-familiar scenario played out when a student opened fire inside Arapahoe High School in Centennial, Colo.

It was just a day before the anniversary of the school shooting in Newtown, Conn., and less than 10 miles from Columbine High School, where the fatal shooting of 12 students and a teacher by a pair of fellow pupils in 1999 was the first in what became a national wave of deadly school shootings.

Police said a student on Friday afternoon fired shots at Arapahoe High School, about 15 miles south of downtown Denver, then killed himself.

Colorado Gov. John Hickenlooper called the shooting an "unspeakable horror."

“We were horrified to hear today of a school shooting at Arapahoe High School in Colorado, just one day before the one year mark of the Sandy Hook shootings," said Tim Makris, executive director of Sandy Hook Promise, a group formed after the shooting at Sandy Hook Elementary School in Newtown last December. "Our hearts are with all the families of Arapahoe High School today." 

At a televised news conference Friday, Arapahoe County Sheriff Grayson Robinson said the student entered the building armed with a shotgun looking for a specific teacher. At least one student was injured, he said. Hospital officials reported one student in critical condition.

A second student suffered minor injuries, but authorities were unsure if that was related to the shooting.

As kids locked down in their classrooms, some took to Twitter to alert their families, while parents posted their own frantic updates.

Kids are running from Arapahoe HS. My heart is in my throat