Colorado Shooting: Three dead in Walmart attack

Three people are dead after a shooter opened fire in a Walmart in Colorado.

Police have warned the public to stay away from the Walmart in Thornton, 16km northeast of Denver, where the shooting occurred just after 6pm Wednesday (local time). Police immediately evacuated the mall when they arrived.

Earlier, Thornton Police said there were "multiple parties down" and law enforcement sources confirmed to ABC News that one person had been killed. However, police have since confirmed the death count has risen to three.

ABC reported that there were two assailants, both believed to be male, who are still on the loose, but police say it is not an active shooter situation. 

"We've got multiple parties down, we're still trying to ascertain what their conditions are," Thornton Police Department's Officer Victor Avila told Reuters.

According to Regional Transportation Denver 48 witnesses have been transported from the Walmart to a nearby police station.

Police have confirmed they are currently interviewing witnesses and reviewing security footage.

A witness told 9 News he heard multiple gunshots while at the self-service checkout and immediately started running. Another said her son heard about 30 shots and helped rescue an elderly woman.

In an interview with Fox News Denver, a Walmart employee said everyone in the store got down and started screaming when gunfire started.

The scene of the attack is within 30 kilometres of both the Columbine High School massacre of 1999 and the movie theatre shooting in Aurora in 2012.

Videos of the scene shows the huge police response to the attack.