WASHINGTON — Pentagon investigators thought they had discovered a major shipment of contraband when they intercepted parts for F-14 Tomcat warplanes headed to Iran, via FedEx, from Southern California. Under U.S. sanctions since its 1979 revolution, Tehran had been trying for years to illegally obtain spare parts for the fighters, which only Iran still uses.

But when agents descended on the Orange County, Calif., home of Reza Tabib, the 51-year-old former flight instructor at John Wayne Airport who sent the shipment, they were astonished to discover 13,000 other aircraft parts, worth an estimated $540,000, as well as a list of additional requests by an Iranian military officer and two airplane tickets for Tehran.

Caught red-handed, the Iranian-born American citizen pleaded guilty in May and was sentenced to two years in prison.

The Tabib tale is among a growing array of cases either under investigation or being prosecuted for illegally exporting sensitive military equipment, from missile parts and body armor to nuclear submarine technology, according to the Justice Department. Many are destined for groups or countries that target the United States and its allies, such as night-vision equipment destined for Iran and for Lebanon's Hezbollah, and components for improvised explosive devices, or IEDs, used against U.S. troops in Iraq.

At least 108 countries have "full-fledged procurement networks that work through front companies, joint ventures, trade delegations and other mechanisms to methodically target our government, our private industries and our universities as sources of this material," Assistant Attorney General Kenneth L. Wainstein said last week.

The Pentagon last year reported a 43 percent increase in suspicious foreign contacts with U.S. defense firms.

The biggest offenders are Iran and China, U.S. law enforcement officials say. Since 2000, U.S. Immigration and Customs Enforcement officers have launched more than 600 investigations into illicit Iranian military procurement efforts and more than 540 investigations into illegal exports of restricted U.S. weapons technology to China. Additional probes have been launched by other U.S. agencies, with overall cases doubling in recent years.

"The majority of U.S. criminal export prosecutions in recent years have involved restricted U.S. technology bound for these two nations," said Justice Department spokesman Dean Boyd. Recent prosecutions involved illegal exports of stealth missile technology, military aircraft components, naval warship data, night-vision equipment and other restricted technology for Iran or China.

Some of the most audacious cases involve Iran. Through reverse engineering, Iranian engineers have produced only about 15 percent of the parts needed for the F-4, F-5 and F-14 warplanes — made famous in Tom Cruise's 1986 "Top Gun" — that have been the mainstay of Iran's air force since the days of its monarchy, U.S. law enforcement officials say. Tehran has also been trying to acquire parts for Boeing 707s; Lockheed C-130 transports; and Cobra, Chinook and Sikorsky helicopters, they say.

This month, Abraham Trujillo and David Wayne were charged in Utah with attempting to export F-4 and F-14 parts to Canada that were ultimately destined for Iran. Last month, Dutch-based Aviation Services International was charged in Washington, D.C., with illegally exporting U.S. aerospace-grade aluminum and 290 aircraft-related components to Tehran. And in July, the founder of Vash International Inc. was charged in New York with illicitly exporting F-5 and F-14 parts.

Airplane components are not Iran's only export target, however. In August, Seyed Maghloubi pleaded guilty in California to plotting to send Iran 100,000 Uzi submachine guns via Dubai.

The China cases are often more subtle, because they are not subject to the same sweeping economic and military sanctions imposed on Tehran since shortly after its 1979 revolution. In June, five members of a Chinese family pleaded guilty in California to conspiring to export Navy warship and nuclear submarine secrets. They were found to have "tasking lists" from Beijing. Two were arrested after a CD with encrypted information on U.S. naval defense technology was found hidden in their luggage before a flight from Los Angeles.

Many export schemes involve parties who have no allegiance to a particular nation or organization, Assistant Secretary of Homeland Security Julie Myers said last week.

One of the biggest recent cases involved ITT, the 12th-largest supplier of defense electronics to the U.S. military. In March, ITT pleaded guilty to a scheme to outsource production of sophisticated night-vision systems to China in violation of the Arms Export Control Act. The motive was enhancing profitability, the Justice Department indicated.

Globalization has facilitated some illegal exports. Trujillo and Wayne made their purchases online using the Internet, and Tabib bought some components for the F-14 Tomcats from the online company that auctions surplus equipment for the Pentagon.

The escalating threat led the departments of Justice, Defense, Homeland Security, State and the FBI to launch an interagency initiative last week that will develop coordinated task forces throughout the United States to investigate and prosecute illegal export cases. But officials say it's not easy. "These are incredibly complicated cases," Wainstein said.

In some instances, cases are brought only because of the export quantities involved. U.S. agents, for example, tracked the sale of the spark gaps used in lithotripter machines to break up kidney stones. Lithotripters need only a few spark gaps, which resemble small spools of thread. But in bulk, they can be used in the trigger for a nuclear weapon.

Asher Karni, an Israeli-South African, was convicted in Maryland in 2005 of buying 200 spark gaps that investigators determined were ultimately bound for Pakistan, a nuclear power.