The man accused of detonating a pipe bomb in a busy Manhattan transit hub on Monday morning had pledged allegiance to the Islamic State terror group, law-enforcement officials have said. Akayed Ullah, who was seriously injured in the attack, told investigators he set off the bomb in retaliation for the U.S. military’s airstrikes on ISIS in Syria, The New York Times reports. He was reportedly inspired by other Christmastime terror attacks carried out in Europe in recent years and had chosen the area because of its Christmas posters, according to the report. While authorities believe Ullah planned the attack after reading ISIS publications online, they said they believe he acted alone. The attack came just weeks after eight people were killed when a driver drove onto a Manhattan bike path in what authorities say was an ISIS-inspired attack.