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Investigators in Nigeria say they have taken 13 people into custody after a mob attacked four students and burned them alive — killings that were videotaped and posted online.

A gruesome video on YouTube that showed the killings led to several arrests, police say. The video shows several men on the dirt, writhing in pain. Some are bloodied. They have tired shoved over their heads.

Investigators not yet determined why the mob killed the men, said Ben Ugwuegbulam, a spokesman for the police in Rivers State, in southern Nigeria.

It’s unclear when the crime took place.

Mob justice is common in Nigeria, with some people choosing to take the law into their own hands because the police are either unresponsive or unwilling to patrol high risk areas after dark.

The posting has drawn comments from some who feel that the police should have done more before the video appeared to investigate the attack.