A South African man arrested Thursday is suspected of playing a major role in the nuclear black market that supplied Libya, according to American and foreign officials. They said arrests and raids in South Africa, Germany and Switzerland over the past week mark a significant turning point in the international investigation of the network.

Johan Andries Muller Meyer, a 53-year-old director of a manufacturing firm in the South African town of Vanderbiljpark, was arrested Thursday and charged Friday on three criminal counts of trafficking in some of the most sensitive nuclear equipment available.

Between November 2000 and November 2001, Meyer "unlawfully and deliberately had equipment that could be used to design, manufacture, develop, expose, and maintain the application of weapons of mass destruction," according to the South African charge sheet.

The charges provide a detailed list of key nuclear weapons components that Meyer's company, Trade Fin, was alleged to be involved with, including: gas centrifuges that enrich uranium for bombs; feed and piping systems that deliver the uranium inside the centrifuges; and a Spanish-made machine that produces the main centrifuge component -- high-precision steel rotor tubes where the enriching takes place.

After months of complex investigations, the International Atomic Energy Agency and partners in about 20 countries are getting closer to understanding the scope of the black market run by Abdul Qadeer Khan, Pakistan's top nuclear scientist, according to government officials and experts involved with proliferation issues. The network is suspected of helping North Korea, Iran and Libya develop nuclear programs.

The officials said they expect more arrests and raids in the coming days. So far, the spate of activity indicates that South Africa was used as a major transit point by middlemen and dealers doing business within the network, the officials said.

"This has exposed an incredible important node of the Khan network, and it is surprising that it has happened in a country like South Africa, which is generally considered a white knight on nonproliferation," said David Albright, a former nuclear inspector and president of the Institute for Science and International Security.

South Africa is often celebrated by advocates of nonproliferation for voluntarily giving up on its nuclear weapons program years ago.

State Department spokesman Richard Boucher commended the country on Friday for "its efforts to act against the A.Q. Khan network."

"We think that the activities that they have undertaken are an important contribution to international efforts to shut down this network," he said.

An IAEA team has been in South Africa for several days helping authorities prepare for Meyer's arrest and worked closely with German and Swiss authorities, who questioned two Germans believed to be directly connected to Meyers.

"The IAEA has been working intensively on its investigation into the network, and countries like South Africa have been sharing important information and cooperating closely with IAEA efforts," agency spokeswoman Melissa Fleming said.

Investigators said Meyer's company imported the equipment, mostly from Europe, modified the items to meet Libya's specifications and then shipped them to other middlemen in charge of making the deliveries to Libya.

According to investigators, who could discuss the matter only on the condition of anonymity, Meyer was doing business with Gerhard Wisser and Gotthard Lerch -- two German businessmen who are also being investigated for their ties to South Africa, Libya and the Khan network.

In a statement, German prosecutors said Wisser and Lerch "are suspected of helping Libya in the development and construction of gas centrifuges in order to produce highly enriched uranium for atomic weapons."

Wisser was arrested last week and is accused of brokering a deal in 2001 to provide Libya with nuclear equipment. According to the German statement, Wisser had been living in South Africa and received more than $1 million to arrange for a South African firm to manufacture the parts. It is unclear whether that firm is the one where Meyer works.

Lerch was named earlier this year by Malaysian authorities as a middleman in the Khan network who tried to obtain equipment for Libya's nuclear program from a South African manufacturer. The name of the manufacturer has not been made public.

His Swiss home was raided last week by authorities searching for documents and accounting records that may show a connection to Khan, according to the German investigators. Neither Wisser not Lerch has been formally charged, and neither could be reached for comment.

Lerch, who once worked for the German vacuum technology firm Leybold Heraeus, has surfaced in a number of nuclear proliferation investigations in Europe. A senior European security official said that Lerch had been investigated in the 1980s for his ties to Khan but was never charged. "It's an old network," the security official said.

Lerch's Swiss attorney, Federico A. Pedrazzini, said they had not spoken in recent weeks, and he declined to address the most recent allegations. Lerch has denied wrongdoing in the past.

The Khan network was exposed last year when Libya announced it was giving up on efforts to make weapons of mass destruction. As part of a deal with Britain and the United States, it agreed to come clean about where it purchased equipment and gained expertise.

Since then, the IAEA and others have been trying to crack the network but have had little success.

Khan, who amassed a personal fortune selling weapon designs to Libya and nuclear components to Iran, was pardoned by Pakistani President Pervez Musharraf -- a key U.S. ally. The IAEA hasn't been given access to Khan, and Pakistan has refused to let the agency do key investigative work in the country, something that has significantly slowed its efforts.

In two recent reports on Iran and Libya, the agency complained that it was unable to confirm details or identify sources without access to Khan or the ability to conduct sampling in Pakistan.

Khan's network operated mostly out of the United Arab Emirates and relied on services and suppliers in numerous countries, including Malaysia, which housed a centrifuge factory. The IAEA believes Khan was working with people in more than a dozen countries and suspected South Africa from early on.

South Africa had nuclear weapons until 1991, when the government voluntarily dismantled six completed bombs. It still has a domestic nuclear energy program and, experts say, the kind of sophisticated manufacturing machinery, expertise and industrial infrastructure sought by those seeking to build nuclear weaponry. South Africa also has one of the world's strictest anti-proliferation laws, with severe sentences and financial penalties for violators.

Timberg reported from Johannesburg. Correspondent Craig Whitlock in Berlin contributed to this report.