In August, a rescue crew in Wichita, Kan., answered a 911 emergency call. A man named Joseph Martier had collapsed inside a dingy storage building at an isolated industrial park just outside town.

Mr. Martier was unconscious from a drug overdose, but he recovered later at a Wichita hospital. It appeared to be just another drug-abuse episode -- except for the drug. It was fentanyl, a lethal "designer drug" that can be hundreds of times more potent than heroin.

The near-death of Mr. Martier, 42, a Pittsburgh businessman now being held on drug charges, helped solve a lethal mystery that had vexed federal drug agents for a year. Since 1991, scores of people on the East Coast had dropped dead after shooting up fentanyl, a drug so strong that a fleck the size of a sugar crystal can kill a healthy adult. Agents had no idea where the drug was coming from.

The nondescript building where Mr. Martier collapsed proved to be part of the country's only operational fentanyl lab, the government now charges. The Drug Enforcement Administration says it is the only active fentanyl lab DEA agents have ever busted.

On Feb. 3, agents who raided the building found chemicals and equipment used to make fentanyl, a heroin-like drug the DEA calls "the serial killer of the drug world."

The same day, agents arrested two middle-aged Wichita suburbanites, each with an intense interest in science and chemistry.

One man -- George Marquardt, 47 -- was a chemical "genius" who had won a state science fair award as a teen-ager but was busted in 1978 for trying to mix LSD with methamphetamines. The other -- Phillip "Sam" Houston, 45 -- was described by friends as an eccentric oil geologist who had built an observatory in his home and unearthed meteorites for a university museum.

Mr. Marquardt was charged with manufacturing and distributing fentanyl, and Mr. Houston with distributing the drug, sometimes called "China white," on the street.

Between them, the DEA now charges, these two amateur chemists were directly responsible for killing most -- if not all -- of the 126 East Coast addicts who died from shooting up fentanyl in 1991 and 1992.

"We can say without a doubt that this lab caused the deaths of many, many people in Philadelphia and in other cities on the East Coast," says Michael Pavlick, a DEA special agent in Philadelphia, where 21 addicts died of fentanyl overdoses between August and October.

Mr. Pavlick says the government may seek life sentences for the defendants under a federal law that covers deaths caused by illegal drugs. Prosecutors will try to match the fentanyl seized in Wichita with samples taken from some autopsies of fentanyl overdose victims, Mr. Pavlick says.

In New York, according to court documents filed there, a DEA chemist has compared fentanyl from Wichita -- provided in December to an undercover agent -- with leftover fentanyl found with overdose victims in New York. His conclusion: The two samples are "consistent."

Some of the Philadelphia junkies died so swiftly that syringes were still embedded in their arms. Almost every day in early September, someone in the city was falling over dead, killed by a massive overdose of fentanyl.

The victims probably thought they were buying heroin, say drug agents in Philadelphia. The street dealers who sold the drug also probably thought it was heroin, they say.

Like heroin, fentanyl is a white crystalline powder that produces an intense euphoria. But unlike heroin, a grain of fentanyl the size of a pinhead can cause instantaneous respiratory arrest.

One form of fentanyl is 80 times more potent than heroin, DEA chemists say. Another form, allegedly produced in the Wichita lab, is 400 times more potent.

Both fentanyl and heroin are diluted and sold in $20 street bags weighing roughly 30 milligrams each. But a safe dose of fentanyl is just one-sixteenth to one-eightieth of a single milligram. To properly dilute a kilogram of pure fentanyl would require 200 kilos of cutting agents, says Anthony Senneca, a DEA special agent in Philadelphia.

"It's very difficult to get the mix just right," Mr. Senneca says. "If you get a hot batch that's not diluted properly, a lot of people are going to die."

Up and down the East Coast last summer, heroin junkies were shopping for heroin but buying fentanyl instead -- and dying for their error.

The DEA was stumbling across strong batches of fentanyl everywhere. In Baltimore, 28 people overdosed on the drug in 1992. Twenty-three died that year in New York, where fentanyl was sold as "Tango and Cash." The deaths spread north to Connecticut and Boston, south to New Jersey, Maryland and Virginia, and on to South Carolina.

Meanwhile, someone was getting rich from making the fentanyl. Agents say a kilogram of fentanyl sells for $240,000 to $640,000, depending on purity. A kilo of heroin sells for $100,000 to $200,000; cocaine, for $20,000 to $25,000. Except for a $10,000 rotary evaporator to dilute the drug, making fentanyl does not require enormously expensive equipment or chemicals.

As the deaths continued, the DEA had been checking purchases of the dozen or so legal precursor chemicals needed to make fentanyl. The undertaking was a massive one. The chemicals are common. They are ordered for legitimate use hundreds of times a day from chemical supply firms across the country.

Finally, there was a breakthrough. Agents came across a suspicious purchase by a Boston man named Christopher Moscatiello. From there, they traced more chemical buys in several states from the East Coast to the Midwest.

Late last year, an undercover DEA agent managed to buy fentanyl in Boston from Mr. Moscatiello, according to a DEA affidavit filed in Pittsburgh. During the yearlong investigation, agents collected 37 pounds of fentanyl in the city.

But even though agents knew Mr. Moscatiello and others were buying chemicals, they did not know where the fentanyl was being cooked. They were able to narrow the list of possible cities to only about 20 across the country, one agent says.

Then Mr. Moscatiello came through. He mentioned to the undercover agent in Boston that his supplier had nearly died from a fentanyl overdose in Wichita.

Wichita? It was an odd site for a fentanyl overdose. The city had never reported a fentanyl death.

There were other curious aspects of Joe Martier's near-death experience. He had not injected fentanyl. He had inhaled its fumes. And he was not from Wichita. He was visiting from Pittsburgh.

Pittsburgh made sense. The city did not have a drug problem nearly as bad as New York's or Philadelphia's, but it eclipsed those cities in fentanyl deaths, reporting 21 such overdoses last August alone.