The reason for the unusual purchase — 220 pounds of highly caustic fluorine gas — was never explained, but someone at Iran’s Sharif University was clearly anxious to collect. For months, the mysterious buyer bombarded a British supply company with telexes, demanding to know when his 45 canisters would arrive.

“We have not received your reply,” complained one telex, sent from the Tehran school’s purchasing department and written partly in broken English. “We are awaiting for hearing from you as soon as possible.”

But the telex, sent in 1992 and made public here for the first time, was not what it seemed. The real purchaser was not a university but a secretive research institute working for Iran’s military. The fluorine gas, investigators later concluded, was to be blended with uranium in a nuclear program that would remain hidden for 10 more years.

The document is part a trove of 1,600 formerly secret telexes obtained by nuclear researchers seeking to unearth the early history of Iran’s clandestine pursuit of nuclear technology. While nearly two decades old, the records offer an unusually detailed glimpse into Iran’s alleged efforts to defy sanctions to obtain sensitive technology — tactics that intelligence officials say continue even now.

Experts who studied the documents say they were struck by patterns of behavior that began early in the program and involved some of the same individuals who run the country’s nuclear efforts today, under the oversight of the same supreme leader, Ayatollah Ali Khamenei, who came to power in 1989. The telexes and other records show Iranians using subterfuge and deception to obtain the parts they needed, and afterward issuing vigorous denials to U.N. nuclear officials, even when confronted with evidence.

“They stick with absolutist lines, and it makes it harder to trust them,” said David Albright, a former U.N. nuclear inspector who obtained the documents and provided a sampling of several dozen copies to The Washington Post.

Iran’s history of concealment and deceit has become more relevant now because of concerns that it is nearing a critical phase in its ability to develop an atomic bomb. Although the government has consistently denied ever seeking nuclear weapons, inspectors have struggled to understand why the Iranians have sought to hide their activities if their nuclear program is, as they contend, solely for peaceful energy production.

A team of technical experts from the the International Atomic Energy Agency traveled to Iran this week to pressure Iranian officials to come clean about past nuclear activities, including alleged research on building nuclear warheads. But the trip ended in failure, with IAEA officials being barred from a key military testing facility.

Iran has repeatedly dismissed suspicious documents about past nuclear work as forgeries. State-backed news media have compared the allegations about an arms program to the unfounded suspicions that Iraq had obtained nuclear weapons before the U.S.-led invasion in 2003.

The documents obtained by the nuclear researchers are similar to telexes seen by inspectors, according to Olli Heinonen, who served as the IAEA’s nuclear safeguards chief until 2005. Heinonen said inspection attempts have faced not only evasive answers but also increased levels of official obstruction.

“Deceit and deception have been a regrettable part of the process,” he said.

The telexes, which cover a period from the late 1980s through the early 1990s, come from a time when Iran was first beginning in earnest to assemble and test components for a uranium enrichment plant. By then, Iranian leaders were already committed to expanding the ambitious nuclear program begun under Shah Mohammad Reza Pahlavi and accelerated near the end of the country’s disastrous, eight-year war with Iraq.

Iranian officials obtained blueprints for gas centrifuges — the machines used to make enriched uranium — from Pakistani scientist Abdul Qadeer Khan and then set about secretly acquiring the equipment they needed from Western companies.

Intelligence agencies routinely intercepted the orders and analyzed them for clues to Iran’s true intentions. Hundreds of the documents were quietly shared among governments as well as with IAEA officials who had access to Iranian facilities and could directly confront Iranian leaders about the purchases. Albright, the nuclear expert, said a Western source provided a large trove of the telexes to his organization, the Institute for Science and International Security, which collects and analyzes data about nuclear weapons programs.

An analysis of the telexes by ISIS highlights the hidden role played by Iran’s Physics Research Center, or PHRC, a now-defunct institute that served as a scientific arm of Iran’s military. U.S. intelligence officials believe the PHRC drove Iran’s secret nuclear research program during its first decade, after which its responsibilities were divided among other institutions. In 2004, when U.N. inspectors asked to visit the PHRC’s former headquarters, Iranian officials razed the building and even scraped away the topsoil around the front lawn.

The telexes confirm what IAEA officials believe was a lavish, global shopping spree that continued throughout the 1990s and beyond. Besides the fluorine gas, Iranian officials ordered mass spectrometers, crucial for analyzing the enrichment level of uranium hexafluoride gas, as well as highly specialized types of motors, pumps, valves and transducers used in manufacturing gas centrifuges.

“The fact that so many items are of the type used in centrifuges, and organized under one specific heading, stands out in the data,” ISIS said in a report analyzing the documents.

Privately, Iranian leaders have responded to evidence of duplicity and deceit by blaming the West, saying the United States and its allies unfairly sought to block Iran from its rightful pursuit of nuclear technology, said George Perkovich, a nuclear expert who has met with senior Iranian officials responsible for the country’s nuclear policy.

“The only way they could get what they need is to keep things secret and use duplicity,” said Perkovich, director of the nuclear policy program at the Carnegie Endowment for International Peace. The Iranian view, he said, is “if we didn’t use these tricks, we wouldn’t get the technology we needed, and to which we have a right.”

Many of the telexes were ostensibly orders from Sharif University of Technology, a prestigious school in the Iranian capital. Yet, the fax number and post office box on the return address belonged to the PHRC, ISIS said in its analysis. An Iranian scientist who headed the military research center, Abbas Shahmoradi-Zavareh, also kept an office at Sharif University. The president of Sharif University at the time was Ali Akbar Salehi, Iran’s current foreign minister and the one-time head of the Atomic Energy Organization of Iran.

One series of telexes that apparently raised eyebrows at spy agencies involved an attempt by Iran to obtain tens of thousands of highly specialized magnets used in gas centrifuges. Fearing perhaps that some of its efforts would be thwarted, Iranian officials sent requests to multiple companies at once, asking for as many as 30,000 magnets made from unusual alloys and cut to precise dimensions.

“Snd us a few samples for testing,” one order, written in the typical abbreviated English used by telex operators, began. “We are looking forward to yr early rply.”

It is clear from the telex exchanges that many of the orders were filled, though in some cases the Iranians were turned down when European company managers became suspicious that the purchase was intended for a nuclear program.

Some of the recipients of telexes forwarded the requests to their governments; others who sold goods to Iran would later contend that they were unaware of Iran’s true plans for the materials. Many items sought by Iran were considered “dual-use” — having both military and peaceful applications — and were not banned at the time.

One official for a German manufacturer of mass spectrometers warned the Iranians in a telex to be careful, saying he could get in trouble if government regulators suspected the parts had a nuclear purpose.

“The purchaser can appear only to be a civilian institution, not military or government,” he cautioned.

The request for fluorine gas was also turned aside, at least temporarily. In February 1992, the British firm that had so frustrated the Iranians with its slow response finally wrote to say that the fluorine canisters would not be shipped. The British government’s export office had denied an export license for the gas, a company official explained.

“As you will appreciate, this decision was outside our control,” the telex read. “We look forward to being of assistance on the future supply of other materials.”