Sam (Bill Pullman) and Elizabeth (Julia Ormond) are really the killlers. They are only posing as F.B.I. Bennett (Kent Harper) accidenly killed Conrad (French Stewart) at the crime scene. Elizabeth kills DeGrasso, Wright and Janet. Sam kills Bennett, Billings and Bobbi (Pell James).
The duo gets away with it, allowing Stephanie (Ryan Simpkins) to survive. Sam was impressed that she was onto to them the entire time because she recalled how they touched each other at the crime scene and did the same at the station.
Bonus: the alternate ending has Sam and Elizabeth dying, with Stephanie and Bobbi surviving.
Thanks John B!