premise: Suspected separatist insurgents shot dead four men and injured three, including a policeman, on Thursday in Thailand's restive Muslim-majority south, police said.
  A 25-year-old Buddhist trader was gunned down at his mobile stall in Narathiwat, one of three insurgency-torn provinces bordering Malaysia, police said, adding that the victim's nephew was also wounded.
  When police rushed to the scene, militants using a mobile phone detonated a bomb near the stall, injuring one policeman, they said.
  Also in Narathiwat, a 46-year-old Muslim man, a former village chief, was shot dead by militants.
  In neighboring Yala province, six gunmen burst into a local government office earlier in the day and opened fire, killing two Buddhist engineers and seriously injuring a woman.
   How were the militants able to detonate the bomb?
hypothesis: killing two Buddhist engineers and seriously injuring a woman