Arapahoe County Sheriff Grayson Robinson addressed the media on Monday to detail further the 80 seconds between when student gunman Karl Pierson entered Arapahoe High School, shot another student and committed suicide in the library.

Investigators say that the 18-year-old Pierson had more than 125 rounds of ammunition when he entered the school with deadly intent.

“He [Pierson] entered that school with the absolute focus on doing the maximum amount of harm to the maximum amount of people that he possibly could,” Robinson told reporters on Monday.

The gunman fatally wounded 17-year-old Claire Davis during those 80 seconds before setting some bookshelves on fire in the library and then killing himself.

Davis succumbed to her injuries and passed away on Dec. 21.

Investigators say that Pierson purchased the shotgun and ammunition legally from a local dealer and was also armed with a machete and three Molotov cocktails on Dec. 13. 

The motive for the attack is still under investigation but Sheriff Robinson pointed to a disagreement the shooter had with a speech and debate coach who has yet to be named by officials. 

Sheriff Robinson says investigators believe Pierson acted alone in the attack. 

Claire Davis will be memorialized on January 1, 2014, at 1:00 p.m. at the National Western Events

Center where Davis competed in a horse show during the 2013 National Western Stock Show and was scheduled to compete in 2014.

Students return to classes at Arapahoe High School next week following winter break.