The Nuclear Regulatory Commission will be at Three Mile Island today, investigating what caused a minor leak Saturday in one of the plant's nuclear reactors, exposing 20 workers to radiation.

Exelon Energy spokesman Ralph DeSantis told "Good Morning America" Monday that they still don't know what caused the leak, which forced the evacuation of 150 workers due to airborne contamination.

"It happened when workers were cutting through pipes and also using a vacuum type device that may have blown radioactive particles up into the air," DeSantis said.

The leak was discovered shortly after 4 p.m. Saturday in an opening of the Unit 1 building that had been shut down for improvements.

Authorities say 20 of the 150 workers who were evacuated Saturday afternoon tested positive for radiation. The worst exposure registered at 16 millirems, the equivalent of about three x-rays.

That employee, officials told ABC News, could be back at work as early as today.

"The actual exposure here does not look like it's going to be a significant health threat to that worker," Union of Concerned Scientists physicist Edwin Lyman told "Good Morning America."

Exelon said this weekend that the maximum occupational dose limit at its plants is 2,000 millirem for its workers.

It took Exelon Energy five hours to notify emergency officials of the leak, but authorities say residents were never in danger.

The incident was, however, a reminder of what could have been -- 30 years ago a partial meltdown at Three Mile Island's Unit 2 was considered the worst accident in U.S. power plant history.

Widespread panic on March 28, 1979 caused 140,000 people in neighboring areas to evacuate and the incident led to sweeping changes in nuclear regulation.

"It could've been much worse and it makes me just want to take preparations," Middletown resident Andy Jacques said after Saturday's incident.

Three Mile Island Leak Raises Questions About Radiation Exposure

Before Saturday's leak, Unit 1 had been shut down for weeks while crews made improvements. The reactor inside was not operating.

"They are still trying to determine what the cause was," an emergency management official said shortly after the leak. "That still remains indeterminate at the time. They do have some errors that they are focusing on."

On a recent tour of Three Mile Island, Plant Manager Tom Dougherty told ABC News' Bob Woodruff that a repeat of 1979 would not happen again.

"These plants are extremely robust," Dougherty said. "There are safety systems that are tested frequently."