The active shooter killed outside a Wisconsin middle school Wednesday was reportedly a 14-year-old student armed with a long gun, sources with the investigation told ABC News.

School officials said the gunman never made it inside Mount Horeb Middle School in Wisconsin, about 20 miles outside of Madison. Instead, the "threat" had been "neutralized outside of the building" following a massive response from law enforcement, according to the district.

An alert for an active shooter outside forced the school into a hard lockdown.

"There has been an active shooter at our middle school this morning. The individual did not breach entryway," the Mount Horeb Area School District confirmed on Facebook.

So far, it's believed the 14-year-old acted alone.

"An initial search of the middle school has not yielded additional suspects. As importantly, we have no reports of individuals being harmed, with the exception of the alleged assailant," the school district wrote.

This is a developing story.