After shipments of innocent-looking cargo caught fire at airports and warehouses in Germany, Britain and Poland over the summer, there was little doubt in Washington and Europe that Russia was behind the sabotage.

But in August, White House officials became increasingly alarmed by secretly obtained intelligence that suggested Moscow had a far bigger plan in mind: to bring the war in Ukraine to American shores.

The question was how to send a warning to the only man who could stop it: Russian President Vladimir V. Putin.

In a series of briefings in the Situation Room, President Biden’s top aides reviewed details of conversations between top officials of the GRU, Russia’s military intelligence service, who described shipments of consumer products that caught fire — in one case, a small electronic massager — as a test run.

Once the Russians figured out how the packages got through air cargo screening systems and how long they took to be sent, it seemed the next step would be to put them on planes bound for the United States and Canada, where they would cause fires as soon as they were unloaded.

While the main concern has been cargo planes, sometimes passenger planes take smaller packages in the free space in their hold.

“The risk of a catastrophic error was clear,” Alejandro Mayorkas, the homeland security secretary, said in a recent interview, “that they could ignite in a fully loaded aircraft.”

In August, Mr. Mayorkas set new restrictions on the inspection of cargo sent to the United States. In October, when the warnings resurfaced, he quietly pressured the top executives of the major airlines that fly to the United States to speed up their steps to avert disaster in the air. Some of these precautions became public at the time; others are not.

But behind the scenes, White House officials were trying to figure out if Mr. Putin ordered or was aware of the plot – or was kept in the dark. And great efforts have been made to warn him to stop doing it.

Reaching for the booklet first developed in October 2022 — when the United States believed that Russia was considering detonating nuclear weapons in Ukraine — Mr. Biden sent his national security adviser, Jake Sullivan, and the director of the CIA, William J. Burns, to send a series of warnings to top aides to Mr. Putin. As one senior official recounted, it took many paths for the message to reach Putin’s ears and sink in.

The gist of the warning was that if sabotage resulted in mass casualties in the air or on the ground, the United States would hold Russia responsible for “enabling terrorism.” Mr. Sullivan and Mr. Burns did not specify what that response would be, but made it clear that it would raise the shadow war between Washington and Moscow to a new level.

That shadow war continues, every day, as Russia turns to sabotage in hopes of breaking NATO’s will to support Ukraine, without starting a full-scale war with the NATO alliance.

It redefined life in Europe, ending the sense of security that came with the post-Cold War world. Now, saboteurs are being hunted hour by hour — in airports, seaports and under the sea, as well as on the streets of big cities like Berlin, Tallinn and London.

But in this case, the warning reached Mr. Putin, the officials said in describing the secret exchanges with the Kremlin for the first time. And they seem to have had the desired effect: the onslaught of fires in Europe has stopped, at least for now. But it is not clear whether Mr. Putin ordered a break or for how long. And it’s possible, officials say, that Russia is using the time to build better, more stealthy devices.

Plausible denial?

The effort to reach Mr. Putin was described by five senior officials interviewed over the past three weeks who requested anonymity to discuss the sensitive national security threat. In recent days, as the administration prepares to leave office in a week, some details of the tense talks with the Kremlin have only just been declassified.

Although officials said their efforts to prevent the worst had been successful, several were clearly shaken. As they leave office, they worry that the Russian military, angered by embarrassing and sometimes deadly Ukrainian attacks around Kursk and other targets inside Russian territory, is now determined to spill the conflict onto European and American soil. But they want to do it using techniques that would not risk an all-out conflict with NATO.

The Russians may have seen the operation as a natural — and, in their view, proportionate — response to Ukrainian attacks on Russian soil, which have depended at least in part on US-supplied weapons, including missiles.

To this day, American officials do not know whether Mr. Did Putin order the operation, did he know about it or did he find out about it only because of American warnings.

Several officials said they suspected the plot could have been the work of GRU officers responding to general orders to increase pressure on the United States and its NATO allies. That would be consistent, they said, with previous efforts to create plausible deniability for Mr. Putin if the operation goes badly.

The incident showed that Mr. Biden and Mr. Putin maintained indirect channels of communication, although they have not spoken since the beginning of the Russian attack on Ukraine in February 2022.

That freeze on direct talks between Washington and Moscow appears set to end: President-elect Donald J. Trump said Thursday that Mr. Putin “wants to meet, and we are arranging it,” although the Kremlin insists there was no official conversation. Mr. Trump and his aides are tight-lipped about whether the two have already spoken. They did not say whether the talks would be limited to the war in Ukraine or would include other elements of the hostile relationship between Washington and Moscow: the looming nuclear arms race, Russia’s future in Syria and an accelerating shadow war with the West.

News of air cargo operations this summer leaked from Europe and The Wall Street Journal reported in early November that intelligence officials he believed Russia’s ultimate goal was to expand operations to the United States and Canada.

But this report is the first to describe how the assistants of Mr. Biden determined that the events could lead to a disaster, if they do not intervene directly with Mr. Putin, even unintentionally, if the plane was delayed due to bad weather or the saboteurs got the time wrong.

“It was a powerful example,” said Mr. Mayorkas, “National Security and Homeland Security Convergences.”

Mysterious fires and cable cuts

In the first two years of the war, Russia seemed determined to keep the conflict within Ukraine’s borders. Its missiles never strayed into NATO territory. On the night it appeared a missile might have crossed the border into Poland and killed two farmers, Mr. Biden was awakened by fears that the two countries would descend into open conflict. To Washington’s relief, it was a false alarm; the Ukrainians shot wrongly.

That changed in 2024. Cases of sabotage and suspected sabotage were popping up everywhere: hard-to-explain warehouse fires, sometimes linked to companies supporting the arming of Ukraine; GPS “spoofing” that paralyzed the navigation systems of ships and flights across Europe; cuts in undersea fiber optic cables where towing anchors from Russia’s “shadow fleet” of barges appeared to be at fault.

Washington helped intelligence officials in Berlin uncover a plot to assassinate the CEO of Germany’s leading arms manufacturer, Rheinmetall. The company is a leading producer of artillery shells that Ukraine desperately needs.

But when an incendiary device caused a fire at DHL’s freight facility in Leipzig, a former East German university city, in late July, it prompted an urgent investigation. Thomas Haldenwang, the head of Germany’s domestic intelligence agency, later told the German parliament that the country narrowly avoided the plane going down, but did not elaborate.

The package was postmark from Lithuania, as was another that exploded in Birmingham, England. The third caught fire in a Polish courier company.

Inside the White House, the biggest concern came in the form of intelligence about conversations among the GRU. US officials would not discuss how they gained access to the conversations. But they confirmed the content: three flammables aimed to determine how DHL and other packages flow, so that the ignition of the highly flammable magnesium-based material can be precisely timed to ignite.

The talks indicated that the next step was to put them on planes to the United States and Canada. But the fear running through the discussions in the Situation Room was that an inadvertent delay – due to weather or the aircraft circling due to heavy traffic – could mean the devices were activated mid-air.

From Washington to Moscow, a warning

In August, the CIA and others concluded that the incendiary devices that exploded in Leipzig, Birmingham and Poland were in fact part of the GRU’s “field work” as it tried to figure out which path the packages were taking on their way through Europe. The packages were sent from Vilnius, Lithuania, where the Russians have a significant intelligence presence.

Associates of Mr. Sullivan recall that he was very focused on the risk of attacks during this period, although he did not say anything about it publicly. But conversations among GRU officials left little doubt where this was headed. One senior official involved in the discussions said it had become clear they had to get the message to Mr Putin, as he was the only one in the Russian system capable of ordering the operation to end. But getting to him meant sending a message in more ways than one.

Mr. Sullivan quietly began a series of calls with his Russian counterpart, Yuri Ushakov, beginning by noting Rheinmetall’s plot. It is not surprising that Mr. Ushakov denied that Russia was involved – just as Russian officials denied, in October 2022, that they planned to use tactical nuclear weapons.

Then, speaking somewhat elliptically about how the United States knew, Mr. Sullivan said Mr. Ushakov that the administration believes the incendiary devices are also Russia’s responsibility — and that they have put civilian lives at risk. A big concern was the risk of mass casualties, he said, if the packages flew off on a cargo or passenger plane.

Mr. Burns, the CIA director who served as American ambassador to Russia two decades ago and is the official who knows Mr. Putin, made essentially the same case to his intelligence equivalent, Sergei Naryshkin, who heads the SVR, and Alexander Bortnikov, director of the FSB, Russia’s two most powerful intelligence agencies. They were all thought to have had regular access to Mr Putin.

American officials were careful not to say the operation was intended to shoot down the plane; in fact the devices appeared to be designed to be activated on the ground. But the risk of an accident in the air seemed great.

Although the immediate crisis was avoided, the assistants of Mr. Biden acknowledges that the incident exposed a larger problem: that as the war nears its third anniversary, the risks are spilling into new arenas and taking on new dimensions.

“As great as a ceasefire in Ukraine is, it’s far from over,” said Richard Haass, a former chairman of the Council on Foreign Relations who has written extensively about what the end of the war might look like.

Sabotage, he said, is “all part of a larger pattern.”

“Russia has turned into a revolutionary actor,” he said. “Russia has turned into a country that wants to undermine the international order. And the real question is: Can the Trump administration do anything about it?