This was reportedly (though unconfirmed) happened when members of the Shia Muslims were protesting against the continued unconstitutional incarceration of their leader (El Zaky Zaky) for over 6 years ago and unlawful training of assault rifles on peaceful protesters.
At least, an estimation of 600 Shia Muslims have been killed by Sunni Islamic forces acting in accordance with order from the executive arm of the Nigerian Federal Government which instigated the massive protests from followers of El Zaky Zaky which all the protests so far were characterized with bloody violence coming from the security agencies.
It is proverbially acknowledged that these actions of security agencies goes against the fundamental human rights especially right to freedom of "religion", "speech", "expression", "fair hearing", "movement", "life", and "liberty".
Nigeria has never been a healthy democratic country since day one but situation is getting worse as religious and political differences grows wild as there is possibility of war breaking out if the situation is not well managed.