Clayton, N.C. — State officials Thursday said they are considering sanctions against the company that operates the yard-waste dump where smoke from an underground fire has been fouling skies over parts of Johnson County all week.

The Department of Environment and Natural Resources cited Stump Dump Inc. in December for covering too much area and for not properly covering waste. A re-inspection had been planned this week.

Instead, firefighters have been tied up at the dump since spontaneous combustion began the fire that has filled the sky with smoke, sometimes raised levels of carbon monoxide in the area and prompted a voluntary evacuation in a three-mile radius.

The fire may continue to burn for days or even weeks, local officials said. Because the burning wood is underground and covers a large area, reaching it with water is difficult.

Use of chemical fire-retardants is being considered, as well as trying to accelerate the burn to use up the fuel faster with less smoke. Emergency management officials said trying to pump water underground was having limited effect on the dump, which reaches depths of 70 feet and contains some large logs and stumps.

Officials said carbon monoxide levels were higher than normal but were within outdoor air-quality limits. They said people should use their best judgment when deciding whether to operate heating and cooling systems. They had been recommending that those systems be kept shut down in homes around the dump, which is off Loop Road.

"We are considering additional enforcement action," said Paul Crissman, chief of DENR's Solid Waste Section. That could mean fines or even being closed, he said.

The smell of the smoke even reached Raleigh Wednesday night, triggering alarms in some  North Carolina State University buildings, campus police said.

A temporary evacuation center at Clayton High School operated Tuesday and Wednesday nights. Johnston officials said they did not plan to operate the shelter Thursday because air quality was not dangerous, but they added that they would use the reverse E-911 system to notify residents if conditions worsened.

Officials had decided at midnight Tuesday to use the reverse 911 phone notification system to tell approximately 6,000 residents in a three-mile radius of the dump on Loop Road about the shelter at Clayton High School. The decision came after air monitoring found unacceptable levels of carbon monoxide.

The precautionary shelter re-opened at 7 p.m. Wednesday, the county said. Three families stayed there overnight Tuesday, they said.

The Johnston County fire recalled a spontaneous combustion fire that broke out at the city of Durham's yard-waste dump last September and burned for two weeks. That fire, which also poured smoke copiously but did not force evacuations, went on until firefighters brought in a chemical fire suppressant. That fire broke out Sept. 10 and lasted until Sept. 24.

State and federal environmental regulators have time-exposure standards for carbon monoxide. One standard involves the amount in outdoor air for an eight-hour period. The other, higher level is for a one-hour period. Neither government has official standards for indoor air.

Johnston County emergency management officials said people with immediate concerns could contact them at 919-989-5050.