(Reuters) - Utah Governor Gary Herbert asked the state's attorney general on Thursday to open a criminal probe into allegations of misconduct at a medical waste incinerator that environmental activists have accused of polluting the air.

The criminal investigation comes in addition to separate probes launched last week by the state's Department of Environmental Quality (DEQ) and its Labor Commission into activities at the plant owned by Stericycle in North Salt Lake.

Herbert's office said the governor had directed that all the investigations be completed as soon as possible.

"If any of the allegations are ultimately substantiated, he intends to use his full authority take swift and aggressive corrective action," the office said in a statement.

Illinois-based Stericycle said it takes safety and compliance issues very seriously, and that it believes Herbert's decision was the result of recent allegations made by an anonymous individual who may or may not be a former employee.

"We believe that many of the allegations are inaccurate and unfounded, and we believe that we have been operating our facility in compliance with applicable regulations," Jennifer Koenig, vice president for corporate communications at Stericycle, said in a statement.

She said similar allegations were made last year concerning public health risks posed by the facility, but that two separate studies by state health officials concluded they were unfounded.

"We have been rigorously and continuously inspected over the past many months and have been found to be in compliance with our permit conditions and applicable regulations," Koenig said.

Environmental activists have long criticized air-quality regulators over the facility, saying it should not have been allowed to continue operating as the area became more populated.

Herbert's office said a probe by the DEQ into possible regulatory violations relating to Stericycle's permits began on Sept. 17, the day the governor's office said it received the allegations.

It said the next day the Utah Labor Commission began studying potential violations of occupational safety and health standards that would endanger workers at the site.

In recent months, it added, the DEQ had required Stericycle to take more steps to better protect surrounding communities, including setting up enhanced emission controls and the installation of continuous emission monitors at the facility.

The plant treats about 7,000 tons of medical waste per year from across the western United States, state environmental officials say.

(Reporting by Daniel Wallis in Denver; Editing by Eric Beech and Sandra Maler)