The small plane's disappearance into icy Lake Constance had all the elements of high intrigue: radioactive materials smuggling, former East Bloc agents, a planned trip to Beijing.

The five people aboard the Cessna 425 - two Berlin businessmen under international police surveillance, two Czech women and their hired pilot - were presumed drowned in the lake's frigid waters.But when the twin-engine Cessna was lifted from the lake bottom last week, no bodies were found. Nor were any of the radioactive substances suspected to have been aboard.

Investigators said the plane made a belly-landing that likely would not have injured its passengers. And it did so after dark. Could the passengers have escaped in a boat? Did they plan the whole thing to shake police off their trail?

On Friday, investigators resumed their search for the bodies.

Today, they said sonar had detected what may be two bodies on the lake bottom 500 feet down. Owing to weather conditions, they said it could be several days before the objects can be brought to the surface with the help of a research submarine.

The developments come as reports in Germany's leading newsmagazines raise a flurry of questions - most of them un-an-swered.

Were businessmen Josef Rimmele, 54, and Klaus Eichler, 53, black marketeers? Or were they hapless amateurs deep over their heads in a shadowy world of weapons and radioactive material smuggling?

Whoever they were, police from several countries had been following their movements. Their disappearance caused consternation.

The Cessna crash landed Jan. 24 in the lake, which is bordered by Germany, Austria and Switzerland and is a drinking water source for several million people. After the crash, German and Swiss newspapers reported that police suspected the plane was carrying radioactive cargo. Bottled water quickly sold out in the region.

Other reports disclosed that Eichler and Rimmele operated out of Prague and dealt regularly with former leaders of communist East Germany and Czechoslovakia's in-tell-igence agencies.

Czech police said they believe the two men were involved in weapons smuggling and money laundering. The German newsmagazine Focus quoted a Prague business partner of Rimmele as saying he was dealing in "Red Mercury" - supposedly a valuable radioactive material.

The Cessna was en route from Paris to Prague when it disappeared. Focus said Monday that Rimmele and Eichler had planned a trip to Beijing - scheduled for two days later - to buy 800 pounds of Red Mercury and 13 pounds of the radioactive isotope osmium-187 for $493 million from China's Defense Ministry.

The two had gotten permission from Russian and Chinese authorities to fly a private plane from Prague to Beijing via Moscow, the magazine reported. It did not name its sources.

In the smuggling underworld, Red Mercury is touted as a key ingredient in nuclear weapons. But nuclear physicists and other experts say it may be a hoax or a code word for something else.

Osmium-187 is not used in nuclear weapons and is of limited value, said Frank von Hippel, a Princeton University nuclear physicist.

Western governments worry that the smuggling of radioactive materials could help terrorists or renegade states build nuclear weapons. But in the flea market of radioactive materials coming out of eastern Europe since the Soviet collapse, little of what authorities have confiscated has proved to be of much value.

The smuggling has been almost exclusively by small-time dealers trading in non-fissionable materials used by hospitals, not weapons-grade material like plutonium, said Hans-Friedrich Meyer, spokes-man for the Inter-na-tional Atomic Energy Agency in Vienna.

Von Hippel, who is helping the Russians secure their nuclear arsenal, said in a telephone interview that Red Mercury "is still a puzzlement to me."