Environmental Protection Agency investigators have uncovered what they believe are widespread violations of federal air pollution laws by coal-fired power plants that have significantly added to the nation's smog problems.

At least a half-dozen electric power plants have boosted the amount of electricity they can generate without getting the required permits and without adding new pollution controls as required under the 1990 Clean Air Act. Dozens more plants are under investigation in what an administration official said appears to be a pervasive sidestepping of the nation's air pollution laws.

"We suspect there is a wide pattern in the utility industry in which a large number of coal plants may have profited by polluting the air beyond the legal health-based limits of the law," said the official, who requested anonymity. "We are doing an in-depth investigation as a result of that suspicion."

Using new computer-assisted investigatory techniques, officials determined that the violations have produced increased emissions of nitrogen oxide--a prime ingredient in smog--ranging from 1,000 tons a year at some plants to as high as 10,000 tons a year at another, the official said.

"The magnitude is huge," the official said, noting that a thousand tons a year is the equivalent of the air pollution produced by about 150,000 automobiles.

Power plants that burn coal are a major source of nitrogen oxide, which contributes to acid rain and produces--in reaction to sunlight--ground-level ozone that can cause respiratory problems. Midwestern coal-fired electric plants are a major contributor to East Coast air pollution because prevailing winds carry pollutants from the West.

If the EPA investigation confirms that violations of the Clean Air Act have been committed on the scale that officials now suspect, the plants could face fines totaling tens of millions of dollars.

Pat Hemlepp, a spokesman for American Electric Power, one of the largest power generators in the nation, said the Columbus, Ohio-based company has been aware for some time of the EPA investigation but that he could not comment specifically on the allegations. "It's been evident for some time that EPA has wanted to pursue an emissions reductions strategy through the enforcement process," said Hemlepp.

Under the Clean Air Act, power companies that make significant improvements, such as adding boilers or turbines that will increase their emissions by 40 tons or more a year, must secure new pollution permits and install state-of-the-art emissions controls.

Exempt under the law and its implementing regulations are improvements that involve routine maintenance, which are covered by existing permits.

EPA enforcement specialists have determined that many of the plants they are investigating have added capacity without seeking new permits, claiming that they are exempt under the routine-maintenance provisions of the law.

Investigators found that pattern at six plants where they conducted on-site field examinations.

They suspect a similar pattern exists at more than two dozen other plants where field work is underway to verify suspicions raised by computer-aided analyses.

The agency has about 100 investigators working on the probe, which could extend to as many as 100 coal-fired plants, many of them in the Midwest and East.

The official who discussed the probe declined to name the six power plants where the agency has found major improvements and no record of new permits because the investigation is still underway.

But the official said it appears the plants have broadly interpreted the routine-maintenance provision as a way to avoid the permit process, and that the practice has been going on for five or six years.

"It looks to us like the industry used the grandfathering provisions on routine maintenance and chose to expand them and create a loophole that goes way beyond what the law allows," the official said. Another official said it is not clear whether the apparent violations are deliberate, and that one goal of the investigation is to determine if that is the case.

The apparently widespread evasion of the Clean Air Act was discovered, the official said, with the help of new computer capabilities that allow agency enforcement officers to track how much power is being produced by individual units within a plant and how much coal is being burned.

That information is then cross-checked with state records on permit applications.

The administration's disclosure of the investigation came on the same day that environmental groups released a report in which they charged that the increasing reliance on coal by the electric utility industry because of deregulation has resulted in large increases in smog-causing pollution during the 1990s.