When you have the ability to detect a "kill team" en-route to kill the President, presumably your goals are to a) prevent the President dying in the first place and b) Apprehend the culprits after the event if they manage to pull it off. With time travel, you're afforded the opportunity to do both, even if they're initially successful.
By placing operatives on either side of the event (and presumably the temporal incursion itself, since it's close enough for them to work out the likely target), they can hopefully catch them before the event or catch them afterwards. Assuming they still get away with their assassination attempt, once their identities and plan are known to the authorities, it should be a fairly simple matter to travel back a second time for another go at preventing the event from ever happening as well as mopping up safe-houses, support personnel, financiers, etc.
As an added bonus, those who're placed before the event will have unaffected memories of what the Prime timeline should look like (e.g. that the assassination was an incursion).