Don't be silly boys - Never lie to the passengers, they are not stupid. The incident has taken place in the cabin and therefore it is likely that some passengers may have already witnessed the scene; the 'rumour' will quickly spread (rather like this site!!!) closely followed by dissent and panic. In a situation like this you need them 'on side' both during the diversion and once stranded at the place of disembarkation.
Most airlines have a 'script' to follow that starts along the lines "Every year, airlines receive numerous security threats which are nearly all false, however, we treat every threat of this kind very seriously indeed..." etc etc.
This PA will come from the Captain and will be coordinated with the cabin crew through a briefing. Remember, it's not necessarily what you say but the way that it is said and (most importantly) the behaviour of the crew in the cabin.