The arrest this month of a businessman accused of smuggling nuclear bomb triggers to Pakistan (search) is the latest sign that the important U.S. ally in the fight against terrorism remains a major player in the nuclear black market.

Asher Karni, 50, is accused of being the middleman for a Pakistani company's purchase of dozens of triggered spark gaps -- electronic devices that can be used to trigger nuclear weapons. Agents arrested Karni on Jan. 2 at Denver International Airport (search).

If the devices were indeed headed for Pakistan's nuclear program, the most likely explanation would be that Pakistan was planning to construct more nuclear bombs. That could complicate Pakistan's relations with nuclear rival India (search).

The United States has restricted sales of nuclear and missile equipment to Pakistan for years because of Pakistan's nuclear weapons program.

Officials from the United States and other governments say Pakistan also was the likely source for at least some of the know-how and equipment for nuclear weapons programs in Libya, North Korea and Iran. Secretary of State Colin Powell said this month that American officials have presented evidence to Pakistan's leaders of Pakistani involvement in the spread of nuclear weapons technology.

Pakistani officials say the government is not involved in any black-market nuclear deals. But Pakistan has questioned three top nuclear scientists in recent months based on information from the International Atomic Energy Agency.

"We have investigated. We haven't come across any evidence" of proliferation, Ashraf Qazi, Pakistan's ambassador to Washington, said Wednesday.

The possible spread of nuclear technology from Pakistan is a greater worry than any attempts by Pakistan to clandestinely supply its own nuclear program, said Robert Einhorn, a former State Department arms control official under President Clinton.

"If we can do it, we should stop both, but clearly Pakistan's export of nuclear materials and technology is a lot worse," said Einhorn, now with the nonpartisan Center for Strategic and International Studies in Washington.

But court documents in Karni's case offer a window on the worldwide nuclear black market.

The deal involved triggered spark gaps, electrical devices whose uses include breaking up kidney stones and triggering nuclear detonations. Anyone exporting triggered spark gaps from the United States to Pakistan must have a license issued by the U.S. government.

Karni heads Top-Cape Technology in Cape Town, South Africa, which trades in military and aviation electronic gear. Karni used an elaborate scheme to try to get around U.S. export restrictions to Pakistan, Commerce Department Special Agent James Brigham charged in a federal court affidavit.

An anonymous source in South Africa tipped off U.S. authorities and provided information, including detailed shipping information to allow tracking of the devices and copies of e-mails and other correspondence to and from Karni, the agent wrote.

Karni's contact in Pakistan asked Karni to try to buy 100 to 400 spark gaps, Brigham alleged in his affidavit. Karni worked to get the devices from an American manufacturer, PerkinElmer Optoelectronics of Salem, Mass.

A PerkinElmer representative in France wrote to Karni last summer that exporting spark gaps to Pakistan would require a U.S. license, Brigham wrote. Karni then contacted a company in New Jersey, which ordered 200 of the devices from PerkinElmer, the agent wrote.

At the request of federal agents, PerkinElmer disabled the 66 spark gaps in an initial shipment to the New Jersey company, Giza Technologies Inc. of Secaucus.

Giza, which has not been charged in the case, shipped the devices to South Africa, listing them on shipping documents as electrical equipment for a hospital in Soweto. Karni repackaged the spark gaps and sent them to Pakistan via Dubai in the United Arab Emirates, Brigham alleged in the affidavit.

While spark gaps can be used in machines called lithotripters to break up kidney stones, even the largest hospital would need only a half-dozen or so, experts say. Orders of large numbers raise red flags with nuclear experts.

A PerkinElmer brochure notes they are useful "for in-flight functions such as rocket motor ignition, warhead detonation and missile stage separation." PerkinElmer's corporate predecessor, EG&G, similarly disabled a shipment of 40 similar devices called krytrons in the 1980s during a sting operation against Iraq's nuclear program.

Karni's case is not the only one involving Pakistani attempts to buy potential nuclear triggers. In 1985, Pakistani citizen Nazir Vaid was convicted in the United States for trying to buy 50 krytrons.

South African police searched Top-Cape's offices last month and Karni acknowledged he had shipped the spark gaps to Pakistan, Brigham alleged in the affidavit.

Under U.S. law, prosecutors would have to prove only that Karni exported the devices without a license and would not have to prove he knew they would be used in a weapons program. Exporting spark gaps to Pakistan without a license is illegal even if the devices are used for health care.

Federal prosecutors are appealing a ruling by a Denver federal magistrate that would set Karni free on $75,000 bond raised by supporters. Prosecutors argue that Karni, an Israeli citizen, should be jailed because he could flee to South Africa or Israel and avoid extradition to the United States.

Karni's Denver lawyer, Harvey Steinberg, did not return telephone messages. Giza's president and chief executive officer, Zeki Bilmen, declined comment.