[MENTION=6776890]amorgan_28[/MENTION]
Good advice so far...definitely an interesting setup that needs a bit more thinking thru. Especially in regards to having 2 separate investigations (1-preventing assassination, 2-catching assassin if assassination succeeds), giving the PCs reason to get involved early, and clarifying the motives/methods of the terrorists.
I would rework your "Preventing the Assassination" outline to something like this:
PCs are a counter-terrorism unit with broad jurisdiction. They get involved when politician begins getting death threats...
1) PCs investigate death threats with booby-trapped letters, find traces of a chemical used in air conditioner manufacturing...this may lead them to the now deserted factory where booby-trapped letters were created. Digging deeper will reveal corporation that owns factory makes deliveries/installs to several location around a city, including locations nearby where politician's speech is planned (hinting at sniper's nest).
2) PCs can try to sway politician to move speech, but stubborn politician won't budge. Investigating locations near speech location where recent AC installs happened may bring PCs across decoy bombs! After narrowly averting death, they can either investigate bomb parts or investigate the shell corporation in charge of the air conditioning.
3a) Ordnance used was very rare military-grade, probably acquired on black market but leaving nagging possibility that someone with great power is bankrolling terrorists. A black market fixer will reveal that the explosives are designed to instantly cauterize wounds (making limb reattachment tricky), a product from an "dirty war" against replicants that can regenerate. A veteran of that "dirty war" happened to purchase a high-powered sniper rifle from the fixer recently, but did so anonymously (fixer only realized he was a vet cause of service tattoo). However, fixer explains that high-powered ammunition is next to impossible to find on black market; you either need to steal it, go out of country, or have powerful friends.
3b) Investigating the corporation...
And so on

The key is to provide easy-to-find clues that have multiple interpretations so that the players need to acquire many clues to piece together the truth.
What's good about the way I've set it up is that it keeps the focus on the PCs but allows for consequences / the terrorists' plot to progress when they fail to make a good deduction and stay one step ahead of the terrorists. I would maintain a sense of urgency to their investigation, as in "3 days before the politician's speech, when he will be most vulnerable..."