An as-yet-unknown intruder who turned up at a Tennessee nuclear plant opened fire on private security guards before fleeing the scene, authorities said Monday.

The incident was serious enough that the Tennessee Valley Authority's Watts Bar nuclear plant went into a heightened state of alert, according to The Times Free Press. The Federal Bureau of Investigation also said Sunday night that it has joined the investigation.

Nobody was injured, but a security guard said that his vehicle was struck by bullets during his exchange with the suspect. Both men reportedly fired several shots.

The guard encountered the intruder around 2 a.m. Sunday, Knoxville-based WATE-TV reported. The alert was cleared after about 13 hours later.

This video is from WATE-TV, aired April 21, 2013.