FBI Special Agent Jerri Williams told ABC News the apparent motive in the Columbus Day attack wasn't to disrupt the plant's operation, but rather to use its computer to covertly distribute mass e-mails or pirated software.

"The concern was high because it is a computer that controls an important infrastructure system, and if, for some reason, it caused it to fail, it would have disrupted service," Williams said.

The hacker originally gained access by tapping into an employee's laptop, officials said. Since the intrusion, the plant has changed all passwords to the system and eliminated home access to the system, ABC said.

WaterISAC, an industry information sharing and analysis center with members from among more than 1,000 water and wastewater systems in the United States, said it was the fourth intrusion into a member's systems in the past four years.

In one attack, the hacker announced entry into the system with the message: "I enter in your server like you in Iraq," the report said.