Foul play in what sense? There's a couple of options: they know/can know where the plane is but say they don't/can't, they got disconnected because of interference or something, the system got turned off, they got hacked, they got hijacked, faulty equipment didn't notify crash, plane fell apart mid-air (well, that means something is faulty, but anyway), plane never existed, plane got shot out of the air,
Just to name a couple, not all equally likely :)
You could even call it on quantum mechanics if you try a bit.