U.S. officials say Ukraine should continue to develop innovative ways to strike at Russian forces as the war approaches its third year. But Ukraine’s use of a Patriot missile to take down a plane last month is an example of how novel battlefield tactics can be fraught with peril as well as promise.

Unbeknown to Ukraine’s military, the Russian aircraft it targeted may have been carrying Ukrainian prisoners of war, according to U.S. officials.

The Patriot is a defensive system, usually used to protect a location and not to shoot down planes. A European partner provided the Patriot interceptor that hit the Russian Ilyushin-76 cargo plane on Jan. 24, according to American officials briefed on the incident.

Russian officials immediately claimed the aircraft was carrying 65 Ukrainian prisoners of war, who were to be exchanged for Russian service members.

Publicly, American officials will not comment on what brought down the plane, though officials who spoke privately on the condition of anonymity said the reports of a Patriot missile being used were accurate.

The question of who was on the plane is less clear. American officials have not confirmed the identities of the passengers, but they said it appeared probable that at least some of them were Ukrainian prisoners. U.S. and Ukrainian officials say Russia may have overstated the number of deaths.

If there were prisoners on the plane, as appears likely, American officials said the loss of life was regrettable.

Ukraine seems to have acted based on legitimate but flawed intelligence. The plane had previously been used to transport missiles, making it a high-value target for Kyiv, according to Western officials briefed on the intelligence.

While the Patriot was fired from Ukraine, the cargo plane went down in Russia. Some American officials say they have encouraged Ukraine to strike far behind the front lines, but only in Ukrainian territory, mindful of the risk of escalation if U.S.-made equipment is used in attacks on Russian territory.

Nevertheless, U.S. officials have not criticized Ukraine for using the Patriot system to target Russian aircraft in general. Instead, they have said this is the kind of innovation Ukraine will need to embrace.

Last month American military planners met with Ukrainian counterparts in Wiesbaden, Germany, to discuss the new tactics that might help change the dynamics of the war, which has been locked in a stalemate for the past year.

One question is whether operations with Patriots are sustainable, especially if Congress cuts off further military aid to Ukraine. The Congressional Research Service estimates that each interceptor costs $4 million. If no more American funding is available, air defense supplies are likely to come under strain.

The Patriot was originally designed to hit aircraft but was re-engineered after the Gulf War to be primarily used to strike tactical ballistic missiles, as those weapons grew in prevalence and importance on the battlefield.

Ukrainians have taken the Patriot back to its origins as an antiaircraft weapon. In January, believing the Russian cargo plane to be loaded with missiles, the Ukrainians set an ambush for it. They moved one Patriot launcher closer to the border, then fired an interceptor when the cargo plan was within range. The tactic, U.S. officials said, are creative but not revolutionary.

Ukrainian officials first devised the plan to use the Patriots against aircraft last spring, in part to try to slow Russia’s aerial bombardment. On May 13, the Ukrainians sneaked a Patriot launcher to an area near the Russian border and took down five aircraft.

After that operation, Ukraine secretly moved a Patriot system to the south, where it was used to shoot down an Su-35, a Russian fighter plane, over the Black Sea.

Only months later, in November, did Ukraine confirm the operations, arguing that the use of the Patriot had, at least for a time, deterred Russian operations.

“They abstained from flying there for a while because they understood it was dangerous, and they could be shot down,” Yurii Ihnat, an air force spokesman, told reporters in Ukraine. “The Patriot system provides such capabilities.”

Over the winter, the pace of operations has appeared to pick up once more. In December, the Ukrainians claimed to have shot down five Russian fighter jets.

President Volodymyr Zelensky said the operations set “the right mood for the whole next year,” promising more to come. While the Ukrainians did not offer details on how they had shot down the jets in December, military analysts said Patriot missiles were probably used.

The commander of a Patriot battery, speaking to The New York Times in December, said the deterrent effect of a Patriot battery was significant.

“When pilots know that there is a Patriot somewhere in the area, they will think 10 times whether to go there to work or not,” the commander said, insisting that only his first name, Volodymyr, be used for security reasons.

While declining to discuss specific operations, he said “the Patriot has already shown its range, a really long range, which allows it to shoot down these aircraft.”

Michael Schwirtz in New York, Anton Troianovski in Berlin, Thomas Gibbons-Neff in London and Eric Schmitt in Washington contributed reporting.