Lagos, Nigeria (CNN) -- 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, and they have tires shoved over their heads.
Investigators have not yet determined why the mob killed the men, said Ben Ugwuegbulam, a police spokesman in Rivers State, in southern Nigeria.
The attack happened Friday.
Mob justice is common in Nigeria, with some people choosing to take the law into their own hands because police are either unresponsive or unwilling to patrol high-risk areas after dark.
The posting has drawn comments from some who say that police should have done more to investigate the attack before the video was posted.