NEW : Authorities are investigating the death and say they do not know the cause . CNN 's Saad Abedine and Mohammed Jamjoom contributed to this report . An opposition group says Bahrain security forces killed the protesters . Thursday marked the second anniversary of major protests in Bahrain .
NEW : Authorities are investigating the death and say they do not know the cause . An opposition group says Bahrain security forces killed the protesters . Thursday marked the second anniversary of major protests in Bahrain .