In the second attack by suspected Islamic extremists on colleges in Nigeria's troubled northeast in the last few days, assailants killed at least 20 students Monday night outside the Federal Polytechnic Mubi, a college in the town of Mubi, WORLD Magazine reports. Some of the victims were shot and others were killed with knives, and a rescue official said at least 20 corpses had been found. Danjuma Aiso, a student at the college, said students were recently warned to leave the college in a statement suspected to have been written by the radical Islamist sect Boko Haram. The attack follows the killing on Saturday of three students outside the University of Maiduguri, about 100 miles away. Boko Haram, which has killed nearly 700 people this year alone in a campaign to implement sharia law across Nigeria, has targeted churches, schools, government buildings and telecommunications infrastructure, but targeting students off campus is new.