On a December morning in 2017, a man left his Brooklyn apartment, entered a New York City subway station and detonated the pipe bomb strapped to his chest in a passageway, terrifying commuters during the early rush hour.

Nobody was killed by the attacker, Akayed Ullah, who later told law enforcement that he carried out the attack “on behalf of the Islamic State.” He had consumed ISIS propaganda on YouTube and scribbled a popular slogan on his visa, passport and a box of bomb-making materials.

Despite that evidence, Mr. Ullah did not provide “material support” to ISIS that morning, according to the federal appellate court that on Tuesday reversed one of his convictions. The court upheld the other convictions he was appealing, which included committing a terrorist attack against mass transportation systems.

While Mr. Ullah will still spend the rest of his life in prison, the decision takes aim at a tool federal prosecutors have used for decades to crack down on terrorism. Some experts say the ruling could affect longstanding convictions and change the prosecution of so-called lone wolf terrorists, including those who have carried out ISIS-inspired attacks.

In cases where a defendant is inspired by but does not directly communicate with terrorists, prosecutors “are going to be concerned that an important tool will be taken off the table,” said Matthew Levitt, a fellow at the Washington Institute for Near East Policy, a think tank. “It could make it harder for some of these charges to stick.”