Classified information, for the eyes of high-ranking government officials only.
Sixty years ago an unidentified ship crashed in the Nevada desert. Some claimed it was extra-terrestrial in origin, some claimed it was nothing more than a hot air balloon. It was neither, it was a time ship of human origin, from some point in our future. Upon investigating the information contained within it became apparent that time travel would one day become available to all. Realising the potential threat from unitentional, or otherwise, alterations to the timeline, the then president ordered a special defence force be secretly created to prevent such accidents.
This is the Time Agency, a force that worked outside of Presidential control, in fact outside of any known legal system. By the time their missions are complete no-one is aware of the alteration to the timeline. Only a handful of people outside of the Agency are aware of its existence, and within the US government only the President knows of it. As for the original ship it was stripped of any proof of its human origins and was last stored somewhere within Area 51.
Emergency memo, date unknown:
Due to a set of unexplained circumstances all contact with the Agency has been lost. All surviving agents are to regroup and attempt to discover the fate of the organisation. This scenario implies there is a sophisticated weapon at work, so be careful