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 . Read more : Why are Nigerians numb to slaughter ? 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 . Read more : Gunmen attack Nigerian university .
A video on YouTube that shows a mob killing four students led to arrests . Graphic video shows bloody victims with tires around their heads . Spokesman said investigators have not yet determined why the mob killed the students .