Iran insists that its nuclear program is entirely peaceful and has even started displaying evidence of Western sabotage. Last September, it mounted an exhibition of equipment it said had been tampered with. The items ranged from pressure sensors and giant industrial pumps to delicate parts for the centrifuges — the tall, silvery machines that spin faster than the speed of sound as they purify uranium, a main fuel of reactors and atomic bombs. The machines are enormously sensitive.

“The exhibition shows only a small part of the hostile measures,” Asghar Zarean, a senior official of the Atomic Energy Organization of Iran, told journalists. The enemy, he added, “is more hostile to us every day.”

Iran’s biggest claim of sabotage centers on its Arak reactor complex, which is still under construction. It is a central issue in the last stages of the negotiations, because if the facility goes into operation, it will create plutonium — a second route to a bomb, and a way to make smaller, often more powerful weapons. Israel has made it clear it will consider attacking the facility the way it destroyed a Syrian reactor in 2007, and the remote site at Arak is ringed by miles of security fences and dozens of antiaircraft batteries.

Mohammad Javad Zarif, Iran’s foreign minister and chief nuclear negotiator, said in an interview with The New York Times last summer that someone had tried to sabotage the reactor’s cooling system.

“It would have caused an environmental catastrophe,” he said, adding that the effort had been detected by Iranian scientists. American officials steadfastly refuse to say if they had anything to do with the operation — but immediately note that if a catastrophic failure had struck the plant, its design and its remoteness would have limited the impact on the nearby population.

If the accord is reached, Iran likely would be able to operate it only at low levels, using a fuel that produces less plutonium.

When it comes to accusations like Mr. Zarif’s, it is often hard to separate fact from propaganda.

But the latest case of covert action was those aluminum tubes, a story not revealed by the Iranians. The details were in a criminal complaint, unsealed in Illinois, against an Iranian identified as “Individual A” who operates from Iran and through “front companies in the United Arab Emirates and Malaysia.”