A gunman took at least two shots at a security officer outside the Watts Bar nuclear power plant in east Tennessee and then escaped in a boat, a spokesman for the Tennessee Valley Authority said.

The incident, which is being investigated by the FBI and local police authorities, occurred just before 2 a.m. Sunday. TVA spokesman Jim Hopson said the gunman has not been apprehended.

FBI officials did not immediately respond to a telephone message left Monday.

The power plant, which was not damaged during the shooting, was put on an "unusual event” status—the lowest caution level for a facility regulated by the Nuclear Regulatory Commission, Hopson said. The facility remained on that status through early Sunday afternoon, he said.

Security at the nuclear plant was not compromised during the incident, he said. The security officer also was not injured.

“But anytime you have shots taken at a security officer at a nuclear plant, that’s a big issue,” Hopson added.

The episode began when a security officer on patrol outside Watts Bar plant noticed an individual standing on the bank of the Tennessee River on plant property. Hopson said the officer challenged the individual, believed to be a white male wearing a hooded sweatshirt.

The individual pulled a handgun and took at least two shots at the officer, Hopson said. The security guard returned fire and called for backup. The gunman got into a flat-bottomed boat and escaped downstream. The exchange occurred in less than a minute, Hopson said.

Hopson declined to identify the security officer.