New intelligence collected from downed Iranian drones in Ukraine shows that a majority of the aircrafts’ parts are manufactured by companies in the U.S., Europe and other allied nations, stoking concern among Western officials and analysts and prompting a U.S. government investigation, according to people familiar with the matter and documents reviewed by The Wall Street Journal.

The documentation of Western parts shows how Tehran has been arming itself and its allies with powerful new weaponry despite being the target of one of the most comprehensive sanction regimes in modern history.

Ukrainian intelligence estimates that three-quarters of the components of the Iranian drones downed in Ukraine are American-made, according to documents reviewed by the Journal. The findings were made after the Ukrainian military downed several drones, including an Iranian Mohajer-6 drone that agents hacked midflight and landed intact, according to Ukrainian investigators.

The components, identified by Ukrainian military intelligence, were verified by the Independent Anti-Corruption Commission, or NAKO, a Kyiv-based nonprofit that inspected the drone. NAKO, whose expertise includes assessing military contracts and arms, showed its report to The Wall Street Journal.

Out of more than 200 technical components identified by Ukrainian investigators that make up the innards of the captured drone, roughly half were made by firms based in the U.S., and nearly a third by companies in Japan, according to the report.

When contacted by the journal, U.S. officials responsible for export control enforcement and the companies whose parts were identified weren’t able to confirm the origin of components or didn’t respond to a request for comment.

The embassies of Japan, Germany, Israel and China in the U.S. didn’t respond or declined to comment.

Iran’s Mission to the United Nations didn’t respond to questions about its use of Western parts, but said that Tehran “is ready to meet with Ukraine at the level of technical experts and investigate drone- or parts-ownership claims.”

U.S. sanctions imposed across Iran’s economy were intended in part to cut the country off from the international financing and trade needed to fund and develop its military, including drones. Similarly, prohibitions in the U.S. and Europe on exports of commercial components that could be used for advanced weaponry are designed to prevent Iran and other foes from gaining from the West’s technical expertise.

The Western-made components that appear to guide, power and steer the drones present a vexing problem for world leaders trying to contain Iranian weapons development and proliferation. Iran’s fleet of unmanned military aircraft, along with the precision-guided missiles they can carry, is now seen by Western security officials as a bigger immediate threat than Iran’s nuclear program.

“A priority is understanding how foreign parts are ending up in Iranian drones,” said David Albright, founder of the Washington-based Institute for Science and International Security, which published its own analysis of Iranian drones last month.

The institute’s report said that besides Western components, there is also evidence that Chinese companies might be supplying Iran with copies of Western commodities to produce the combat drones.

The U.S. federal agency responsible for enforcing export controls, the Commerce Department’s Bureau of Industry and Security, launched an investigation into the Western-origin parts, according to industry officials familiar with the matter.

A senior Commerce Department official declined to comment on any particular situation but said “the proliferation of weapons into Ukraine for use against Ukrainian people is a top priority for us, and we’re going to investigate any illegal export that might be connected with that effort.”

Russia began using Iranian drones to attack critical infrastructure after a series of Ukrainian battlefield successes through the summer months sent its Russian forces into retreat. Tehran’s Shahed-136 model caught immediate public attention when Russia began using the unmanned aircraft, dubbed kamikaze drones because they strike targets by crashing into them, rather than launching weapons.

Ukraine has been pummeled by drones that it says are made by Iran and supplied to Russia.

Photo: Roman Hrytsyna/Associated Press

The Western-made components underscore the difficulty authorities have trying to stem Tehran’s proliferation of its drones.

Many of the parts aren’t under export controls, and can easily be bought over the internet and shipped to Iran through other countries that draw less attention, according to industry and Western security officials. Such transshipments are a violation of law, though they can be difficult to prevent, those people say.

Tuesday, the U.S. Treasury sanctioned several Iranian, Russian and United Arab Emirates companies and individuals it said are responsible for the production and transfer of the Iranian drones used by Russia in Ukraine.

The servomotors in the Mohajer-6, which allow operators to maneuver the unmanned craft through the air, were made by Japan’s Tonegawa-Seiko Co., according to Ukrainian intelligence documents and NAKO’s report.

The firm didn’t respond to a request for comment.

Ukrainian investigators say they found in the downed Mohajer-6 nearly a dozen servomotors (in blue here) made by Japan’s Tonegawa-Seiko, devices used to navigate the drone through the air. Nearly identical models of these devices can be purchased through online hobby stores.

Photo: NAKO

Japan’s Trade Ministry last year charged the firm for exporting servomotors to China without a permit after United Nations investigators found one of the parts in an Iranian drone. The company told local media that it didn’t know they would be used in military drones.

A host of other electronic components were manufactured by units of German-owned Infineon Technologies AG and Arizona-based Microchip Technology Inc., two of the world’s top-tier chip manufacturers, according to the intelligence and NAKO report.

Brian Thorsen, a spokesman for Microchip Technology, said the firm “takes care to maintain supply-chain integrity,” which includes screening clients. He also said that besides having more than 120,000 customers in the industrial, aerospace, defense and other sectors, third-party distributors also sell its products around the world.

“Without access to the device itself, we are unable to advise whether it is a Microchip product or counterfeit product, and if it is a Microchip product, how it ended up in this particular application,” Mr. Thorsen said.

An Infineon spokesman said the company doesn’t sell any of its products to Iran.

The high-resolution telescopic infrared lens used in the Mohajer-6 for surveillance and targeting appears to be identical to a model made by an Israeli firm, Ophir Optronics Solutions Ltd., according to photos of the device in the Mohajer-6 and corporate brochures reviewed by the Journal. The company declined to comment.

The infrared camera discovered in the Mohajer-6 is used for target acquisition and is an export-controlled item because of its military applications, industry officials say.

Photo: NAKO

MKS Instruments Inc., Ophir’s parent company, said that it doesn’t sell to Iran and that it abides by U.S. or other applicable laws, including export controls and sanctions.

Identifying the origins of a particular component can be difficult, even for the companies which produce them.

For example, Ukrainian intelligence suspected the infrared camera was manufactured by Sierra-Olympic Technologies Inc., an Oregon-based company that uses Ophir lenses, according to the report. But Chris Johnston, Sierra-Olympic’s founder and chief executive, said after reviewing photos that some of the camera’s parts are the same, but others are different, suggesting that it didn’t originate from his firm.

Sierra-Olympic had followed up with every client that bought one of the cameras after being contacted by the Journal, “and every one is accounted for,” Mr. Johnston said.

Mr. Johnston said his firm declined purchase requests made several years ago by an Iranian national and alerted federal authorities. But the Iranians could have obtained parts from Western military aircraft downed in Iraq and Afghanistan or from intermediaries, he said.

“The holes in the export laws are primarily people who are bad actors, bad actors who refuse to obey U.S. export laws and those of their host countries,” Mr. Johnston said. “If someone in the U.S. bought something export-controlled and shipped it on to Iran, they’d be in jail.”

Parts of an unmanned aerial vehicle, what Ukrainian authorities consider to be an Iranian-made Shahed-136, in Kyiv after a Russian drone strike in October.

Photo: Vladyslav Musiienko/REUTERS

Taimur Khan, the lead Iranian researcher at Conflict Armament Research, a private company based in the U.K. that investigates global arms trafficking, said companies should do a better job of ensuring their products don’t end up in questionable hands.

“Tracing the items down the chain of custody is crucial,” he said. “It is the responsibility of every intermediary in the supply chain to know its customers and assist investigators in order to identify the acquisition networks.”

Another problem analysts point to is that while the U.S. has imposed sanctions against Iran, most other governments, including European nations, haven’t.

Top Treasury Department officials have warned in recent weeks they plan on sanctioning the procurement channels Iran is using to outfit its drones. But some industry experts say those efforts aren’t enough.

James Rogers, an associate professor of war studies at the University of Southern Denmark, said it is time for the U.N. to develop new rules to prevent the spread of drone technology to hostile actors around the world.

“This is a global problem,” he said, “and it requires a global solution.”

—Dion Nissenbaum and Benoit Faucon contributed to this article.

Write to Ian Talley at Ian.Talley@wsj.com