Pennsylvania officials were alerted to prepare evacuation plans yesterday as federal and state investigators began combing a five-square-mile area for leaks of lethal hydrogen cyanide gas from an illegal underground dump site.

Officials at the site near Pittston in northeast Pennsylvania said they were fearful that an unknown quantity of cyanide dumped into a sprawling abandoned coal mine would combine with acidic waste water and form the lethal gas.

Investigators began what they said would be a month-long search to check hundreds of holes drilled into the mine from homes and businesses. The holes are used for illegal waste disposal, and officials said they were worried the gas -- if it is in the mine -- would seep out through them.

"As far as we know, the gas is not leaking into the air," said Thomas Massey, the team coordinator for the U.S. Environmental Protection Agency. "If it was leaking, we'd know because there would be people dead."

Officials stressed yesterday that they had no evidence that the gas had formed or that it was leaking from the mine into the air. But they said they knew that a "significant amount" had been dumped illegally into the mine at a site in Dupont, about three miles from the mouth of the mine, and that traces were found yesterday in waste water leaking from the mine mouth.

Federal sources said yesterday that the first indication that there is cyanide in the mine came from information gathered recently during a state grand jury probe in Philadelphia of illegal waste dumping.

Late last week the Pennsylvania Department of Environmental Resources, without identifying cyanide, said "significant amounts" of toxic chemical had been dumped into the mine from November 1978 to last July. Federal sources said the cyanide is part of more than 100,000 gallons of toxic waste dumped into the mine through a borehole at a gas station in Dupont.

A man identifying himself as the son of the owner of the station, reached by telephone yesterday, said, "We don't give no information out to no newspaper reporters because they're no good," and hung up.

More than 80 state and federal investigators, wearing emergency breathing equipment, began checking the ground above the mine yesterday at 1,000-foot intervals. The investigators will spend more than a month on the search testing hundreds of existing boreholes into the mine and drilling others, officials said.

"We're still dealing with the unknown at this stage," said Massey. "We know what was dumped in there, and we believe there is gas somewhere in the mine, but we don't know what will come out."

Gov. Richard Thornburgh and state civil defense officials were notified of the potential hazard and the possiblity that some evacuations could take place if gas is discovered, federal and state officials said.

About 50,000 live in five townships atop the mine, according to James Chester, regional director for the state departemtn of environmental resources.

Chester said that despite the potential hazard from the gas, investigators were having problems gaining access to the boreholes into the mine because residents feared they would be sealed and there would be no way of disposting of sewage.

Local officials struck a bargain with the investigators yesterday, however, granting access to the boreholes in exchange for a promise that there would be no legal action against them.

Chester said that investigators testing runoff the mine into the Susquehanna River yesterday found cyanide at levels about 50 parts per billion.

Danger from the cyanide in the water runoff from the mine is "minimal," Chester said. But he said that conditions inside the mine are so hazardous because of toxic waste dumped there that sending investigators inside the shaft is impossible.