MUNICH, Germany -- German officials said yesterday that police had made the biggest seizure yet of weapons-quality nuclear materials smuggled from Russia, calling it the most unsettling indication to date of a well-organized criminal conspiracy to provide buyers with the ability to build a bomb.

As much as 500 grams, more than a pound, of highly radioactive plutonium 239, the prime fissionable material of atomic warheads, was seized Wednesday from baggage from a Lufthansa flight originating in Moscow at Munich international airport, the officials said.

Three men -- two Spaniards and a Colombian -- believed to be couriers were arrested, an official said.

"Further details cannot be revealed because of ongoing investigations," the Bavarian state police said in an announcement.

What the German authorities are most worried about, an official said, is that maverick Russian scientific or security personnel could be masterminding an operation to sell stolen bomb-grade materials to foreign countries or terrorist organizations that want them for building nuclear weapons.

A Russian deputy minister of atomic energy, Viktor A. Sidorenko, was also a passenger on the plane, said a report by the newsmagazine Der Spiegel, which said he was coming to talk with Bavarian officials about civilian nuclear projects.

But neither police announcement nor other officials asked about the case mentioned him, and police spokesmen would not say whether they were investigating a possible connection with the three passengers who were arrested. A federal government spokesman refused to comment.

Gennadi N. Bogachov, a duty officer at the Russian Ministry of Atomic Energy, said yesterday, "No losses of plutonium have been registered at any of our facilities."

Mr. Bogachov said that the Russian government was not persuaded that the plutonium found in Germany was in fact Russian, adding, "We have very tight security at all our facilities."

He confirmed that Mr. Sidorenko was on the same flight to Germany as the suspected couriers, but said there was "no connection" between them and Mr. Sidorenko. He said that Mr. Sidorenko had flown to Germany on official business, but that he did know his exact purpose or his whereabouts.

The seizure Wednesday was the third that the German authorities have announced this summer involving small amounts of enriched radioactive materials capable of being used in nuclear warheads.

The two earlier ones involved much smaller amounts of fissionable materials said to have been samples provided from Russian sources.