It was just after 9 a.m. on Monday when lights started flashing on an isolated sensor inside one of the buildings at Forsmark, a four-reactor nuclear power complex north of Stockholm.

Triggered by radioactivity on the shoes of a worker, that alarm appears to have been the outside world's first indication of a major disaster at a nuclear reactor in the Soviet Union. The radioactive air the accident sent across the Baltic Sea to Scandinavia has left this country's soil contaminated with toxic materials that will take decades to decay.

But for many hours after the initial signal, neither Sweden nor the rest of the world knew where the radioactivity had come from. Left in the dark by Moscow, the Swedes were forced to begin a nuclear detective hunt, activating detection, analysis and civil defense systems that had been tested often, but never proven in a real emergency.

When the alarm first went off, Forsmark safety inspector Bengt Bellman was concerned because the area in question was outside the reactor's restricted zone, where low levels of radiation could be expected. When nothing was found in the air around the sensor, workers who had passed by it were summoned and checked with a hand-held measuring device.

High radioactivity registered on the soles of one worker's shoes.

"My first thought," Bellman said, was that the man had come from a restricted zone, and "had been careless with safety routines" on leaving.

But the man had not been into any restricted areas that morning. It appeared that he had brought the radioactivity in with him from outside the plant. Inspectors were sent to measure levels inside the fence surrounding the reactors.

"To our astonishment," Bellman said, "we found high radioactivity at several places on the ground." When the results reached Forsmark chief Karl Erik Sandstedt, he called senior officials in Stockholm. He was told to take additional readings inside the buildings, where no unusual radiation was found, and outside the fence, where levels were high.

Based on that information, officials decided there could be a leak at Forsmark. Sandstedt immediately ordered the plant's 700 workers evacuated, and authorized local radio warnings for people to stay inside.

In Stockholm, the National Radiological Protection Board and the Swedish Nuclear Inspection Board met in emergency session along with government officials.

Although they suspected a leak, officials initially were puzzled that high radioactivity had been found outside, but not inside, the Forsmark plant. Within an hour, however, a radiological board official recalled later, "we had reports of similarly increased activity from Studsvik," the site of a research reactor in southern Sweden.

For the first time, radiological board official Hans Ehdwall said, "we started to realize that it was perhaps something outside of Sweden that was causing it."

Inspectors were sent out to read the unmanned radiation monitors scattered around the country that are part of Sweden's civil defense system. They showed that radiation levels had begun to rise at about 2 p.m. Sunday in a progression that started in the southeastern part of the country. By Monday, they had reached five, and in some cases 10, times the normal radiation background levels. It was at that point, Ehdwall said, that they knew for certain "it wasn't Forsmark."

The next call was to the government meteorological center to check prevailing winds for the past several days. The winds had come from the southeast, across the Baltic.

By lunch time, the Swedish Foreign Office had contacted its embassies in Moscow, Warsaw, Berlin and Helsinki to gather information about a possible nuclear leak.

In Moscow, the Swedish Embassy received assurances from the Soviet Atomic Power Inspection Board that there had been no reactor accident in the Soviet Union. All the other replies were also negative.

Meanwhile, Sweden's Scandinavian neighbors began reporting readings of high radiation levels. Tests on components of the radiation here showed at least 16 different radioactive elements, some of them considered dangerous even at low levels. The components seemed to be from a civilian rather than a military source.

Outside the official meetings in Stockholm, most of the Swedish public knew little of what was happening until the afternoon newspaper Expressen published an extra edition. Based on reports from the area around Forsmark, and still operating on the assumption that a reactor there had leaked, its headline screamed, "Crisis Meeting After Alarm About Radiation."

Although it was decided that the situation did not yet merit a nationwide alert, the government began taking precautions to cope with either a nuclear leak somewhere in Sweden, still considered an outside possibility, or one from another country. Private cars leaving reactor sites were measured for radioactivity, as were incoming trucks on the ferry from Poland. Analysis of the components of the radiation continued.

Based on the Expressen extra and radio reports, Swedes began jamming the national emergency telephone number, 90000, to ask whether they should stay indoors, refrain from drinking water or take iodine tablets to counteract radiation poisoning. Even physicians were calling.

The issue of nuclear power is an extremely sensitive one in Sweden. In a nationwide referendum six years ago, the Swedish public voted to phase out all nuclear power reactors by the year 2010. Although Prime Minister Ingvar Carlsson has pledged to implement the vote, it is a divisive issue even within his Social Democratic Party. At least one of the leading center-right opposition parties is against the end of nuclear power, as is much of the business community.

Nuclear power is a similarly sensitive issue with two of Sweden's closest neighbors, Norway and Denmark, which have no nuclear programs. The Danes have long been concerned about Sweden's Barseback power station, barely 10 miles from Copenhagen across a narrow band of water, and have asked for it to be closed.

Civil defense officials began checking their supplies of protective masks, and found they were several million short of the country's population. Evacuation shelters around the country were opened for inspection, and it was discovered that many were being used by the government as overflow storage facilities.

Although officials here had strong suspicions, despite the Soviet denial, it was not until the evening television news in Moscow that Sweden and the rest of the world discovered that there had, in fact, been a nuclear accident in the Ukraine.

Anger and concern here were only slightly tinged with relief at discovering the answer to the mystery. Throughout Monday night, Swedish nuclear officials worked on a long list of "specific and highly technical questions" about what had happened and was continuing to happen at Chernobyl to be transmitted to the embassy in Moscow. The questions were handed over to Soviet authorities early Tuesday morning. The Swedes still have received no comprehensive answer.

It was also Tuesday morning when Moscow delivered its first confirmation of the accident in a bilateral meeting.

"We received today a visit from the technical attache at the Soviet Embassy," Bengt Petterson of the Nuclear Inspection Board told reporters that afternoon. "The Russians want advice on how to put out a fire."