Seems odd for a committed terrorist, they're usually sort of mercilessly absolutist. Odd. There's more to this story.
Think about how they’ve handled other “hostage” situations....regarding terrorists. Officials know the situation could become explosive any time....
The number of the police and the firetrucks etc. coming in evidence their coul be other happenings they’re preparing for.
If it’s true there are explosives, and I wouldn’t be surpprised...then this would account for the engines.
Maybe the gunman was buying time by letting those three go?