WASHINGTON — An overloaded computer network prompted an emergency shutdown in a reactor at the Tennessee Valley Authority’s Browns Ferry Nuclear Plant in Athens last year, and federal regulators still can’t say where the interference came from.

The shutdown occurred amid growing congressional scrutiny over whether utilities and other high-risk sites are vulnerable to cyber attacks as they increasingly rely on computer networks to remotely control their operations.

Nuclear Regulatory Commission officials say the August 2006 malfunction did not threaten the safety of the plant and that they are confident an outside hacker was not responsible. But in a letter to the agency this week, the House Homeland Security Committee called for a broader investigation, citing a host of unanswered questions.

Browns Ferry spokesman Craig Beasley on Thursday took issue with the AP’s characterization of the Unit 3 shutdown as an “emergency.”

“It was not an emergency shutdown,” said Beasley. “The operators saw a problem and chose to shut the plant down.”

Beasley said the problem was a result of poor energy supply to the recirculation pumps.

“The pumps weren’t working as they should and it showed up on the control panel, so we shut down the plant manually,” said Beasley.

Beasley said the solid-state power supply to the pump motors is operated by a computer controller.

“This network became overloaded, and when it got overloaded it couldn’t process the information so the solid-state stopped supplying power.”

Beasley said technicians “basically segmented controls apart to preclude any kind of overload again…They’ve been working fine since. We made sure it wouldn’t happen with Unit 2.”

Beasley stressed that the control system is not connected to a network outside the plant. “It was excessive internal traffic,” said Beasley.

“It appears from the information that we’ve collected so far that this (plant) may or may not have been compromised. We want the NRC to determine the source,” committee Chairman Bennie Thompson, D-Miss, said in an interview.

“We need to know whether instances like this are internal or external, and to what extent we are going to deal with them. For the NRC to rely on the operator’s explanation of what happened ... we think does not go far enough,” he said.

In a report issued last month, the NRC said TVA officials manually shut down the plant’s Unit 3 reactor after “excessive traffic” on the computer network caused recirculation pumps to fail, creating a potentially unstable condition.

Although TVA hasn’t determined the source of the data overload, the NRC said the utility reacted appropriately to the failure and has addressed it by installing new “firewalls” to better control traffic on the network.

NRC and TVA officials said the Browns Ferry network involved is an internal-only network and — when operated as designed — cannot accept data from outside sources. TVA spokesman Terry Johnson said the utility believes the failure may have been caused by an unexplained glitch in the computer system.

But when pressed, the officials would not categorically rule out the possibility of outside access.

“We have reasonable assurance that there is no external access to this system,” said Eva Brown, the NRC’s project manager at Browns Ferry. “We did an independent assessment to convince ourselves that (TVA’s) conclusions were acceptable, and there was no evidence of an external source.”

Shutdowns at nuclear plants are somewhat rare; Browns Ferry had two shutdowns in all of 2006, and has had two so far this year.

NRC spokesman Scott Burnell said the agency’s public notice on the August 2006 incident should serve to warn other operators of the potential problem, although the NRC is not requiring any action.

“At this point there isn’t any regulatory reason to,” he said. “Sometimes it does take small events like this to bring issues to the attention of the staff at the plant and the NRC. That’s why we issued this informational notice.”

Joe Weiss, managing partner at Applied Control Solutions and an expert on industrial computer security, said he doubted that anyone intentionally caused the Browns Ferry network to fail. But, he said, it raises concerns regardless.

“The whole area of cyber security in industrial facilities is effectively in its infancy,” he said. “There needs to be a greater appreciation within the nuclear community that these systems truly are connected.”

Since the Sept. 11, 2001, terrorist attacks, security experts have warned of vulnerabilities in the computer networks of the nation’s “critical infrastructure,” including emergency response agencies, electricity providers and water treatment plants.

A 2005 report from the Environmental Protection Agency’s inspector general, for example, found that water utilities had installed computer-based remote controls “with little attention paid to security,” leaving valves, pumps and chemical mixers open to cyber attack.

In 2003, a computer virus temporarily disabled the safety monitoring system at the Davis-Besse nuclear station in Ohio, even though the utility thought the network was protected from such a breach.