SAN ANTONIO – A security guard is under investigation after he shot and killed a man Sunday morning at a West Side bar.

According to authorities, the guard and a customer were arguing inside the bar when it continued outside to the parking lot. Witnesses told police that's when shots were fired.

When police arrived at the in the 7900 block of Culebra Road they found one man dead.

The security guard told police the man shot at him first, so he returned fire, hitting and killing him.

Police are now investigating whether or not the shooting was an act of self-defense.