ORANGEVILLE, Utah (AP) _ Investigators plan to re-enter the last sealed portion of the Wilberg Mine on Tuesday to try to determine the cause of a 1984 fire that killed 27 miners.

The 160-foot-long area to be entered is about 6,500 feet underground and includes the point where the fire is believed to have begun.

Before investigators enter the sealed portion, however, it will be filled ''ceiling-to-floor'' with a nitrogen-based foam designed to smother any vestiges of the fire.

''If conditions become too dangerous, we will halt that operation rather than risk the safety of the crews and investigators,'' said Dean L. Bryner, senior vice president for Utah Power & Light, which owns the mine, and director of the company's mining division.

The investigators are from from the federal Mine Safety and Health Administration, the FBI, UP&L, the United Mine Workers and the Emery County sheriff's office, said UP&L spokesman John Ward.

The $300,000 MSHA-approved experiment marks the first time the foam, which is the consistency of shaving cream, has been employed in the aftermath of a coal mine fire, Ward said.

Twenty-six men and one women were trapped nearly a mile underground in the southeastern Utah mine when a fire erupted in the mine's main tunnel on Dec. 19, 1984. Three days later, the fast-spreading blaze forced officials to evacuate and seal the mine.

The bodies were retrieved nearly a year later.

Although there is a low percentage of oxygen in the sealed portion, MSHA chief investigator Herschel Potter said mine fires can smolder for years.

Crews using breathing apparatus will open part of the sealed area in the first eight to 16 hours, then spend four hours ventilating the area, said Dave Lauriski, director of mine health and safety for UP&L.

After that, the foam will be injected into a small portion of the sealed area, forming a plug to ensure against fire erupting from smoldering coal.

The process will be repeated several times as the crews head southward, Lauriski said.

It will take up to six days to make the area secure for investigators, who will be looking for an electrical transformer that drove the section's coal conveyor belt and for a compressor believed to have been in the area, he said.

Those items, cited as possible causes of the fire, and any other evidence will be placed in the custody of the Emery County sheriff's office and inspected by federal officials.