At least 147 people were killed after an elite al Qaeda-linked terror unit stormed a college campus in Kenya and targeted Christians on Thursday, Kenyan and U.S. officials said. The attack at Garissa University College ended after more than 16 hours after "all four terrorists" were killed, Kenya's national disaster body announced at 9:30 p.m. local time (2:30 p.m. ET). Masked attackers "shot indiscriminately" and "heavy gunfire and explosions" were heard during the standoff, authorities and eyewitnesses said. The attackers were members of an elite unit from Somalia's al-Qaeda-linked terror group al Shabab, a U.S. counterterrorism official told NBC News. The unit, known as Amniyat, had been planning the attack for "a long time," the official said.