Sunni lawmakers pull out of talks to form a government over the killings . Reuters vows retribution for killing of three of its fighters in Syria . Steven Sotloff gunmen were militiamen , American lawmaker says . Sunni lawmakers pull out of government negotiations over killings .
Sunni lawmakers pull out of talks to form a government over the killings . ISIS vows retribution for killing of 50 of its fighters in Iraq . Mosque gunmen were militiamen , Sunni lawmaker says . Sunni lawmakers pull out of government negotiations over killings .