MAIDUGURI, Nigeria (AP) — Assailants shot and stabbed to death at least 20 students in the second attack suspected to be carried out by Islamist extremists on colleges in Nigeria's troubled northeast in the last few days, authorities said Tuesday.

Some of the victims of Monday night's attack were shot and others were killed with knives just outside the Federal Polytechnic Mubi, a college in the town of Mubi, said a rescue official who is not allowed to speak to press on the issue. He said that at least 20 corpses had been found.

Danjuma Aiso, a student at the college, said assailants attacked rented accommodation outside the campus between 10 p.m. Monday and 3 a.m. Tuesday and that he fled to the university campus.

He said students were recently warned to leave the college in a statement suspected to have been written by the radical Islamist sect known as Boko Haram.

The attack follows the killing Saturday of three students outside a university campus about 100 miles away in Maiduguri, Boko Haram's spiritual home.

Ahmed Mohammed, a spokesman for the University of Maiduguri, said Monday the university was aware of the attack but that he could not comment as it occurred off school premises.

Boko Haram has launched frequent attacks in Maiduguri and, to a lesser degree, in Mubi.

The extremist sect claimed responsibility last month for the destruction of more than 30 phone towers across Nigeria's north, including Maiduguri and Mubi. Those attacks left at least two dead in Mubi, police said, and created communications chaos in a nation that relies on mobile phones.

Boko Haram's deadly campaign has targeted mosques, churches, schools and government, security buildings and more recently multi-million dollar telecommunications infrastructure, but targeting students off campus is new.

Boko Haram's name means "Western education is sacrilege" in Hausa and the group has been blamed for killing more than 690 people this year alone, according to an Associated Press count. The group wants the federal government to release its imprisoned members and implement strict Shariah law across Nigeria, which is largely divided into a Christian south and a Muslim north.