At least one person killed and three more injured after an unknown assailant opened fire in Austin, Texas.

At least one person killed and three more injured after an unknown assailant opened fire in Austin, Texas, Sputnik reported.

Earlier, authorities reported multiple casualties following the attack.

This is an active shooter situation, according to local police.

"Active shooter incident downtown, multiple victims. Stay away from downtown," the police wrote in its official Twitter account.

Paramedics report that wounded people are spread all over the area of the attack, according to Reuters.