Why do you find it impressive? I have a distinct feeling they know much more then this... I'd be surprised if they didn't have by now a file of at least half of them. What we've seen is simply competent police work, nothing more. The police are trained, paid and well funded to do exactly this kind of thing... I'm really not surprised to find it worked for once.

I am surprised though to see it public. In the good ol' days this would have been handled much more discreetly.

The first lead probably came from the hotel cams.

  When al-Mabhouh arrives at his hotel around 3 p.m. on the 
  19th, the footage captures two of the suspects, dressed in 
  tennis gear, getting into the same elevator with him to 
  follow him to his hotel room. The two suspects later 
  checked into the room across the hall from him

It's not that impressive if the tapers knew where the suspects were going before hand.

They probably got it from the secret service? Those likely have most of this information in a database and can get it with the the press of a button.

That would be a pretty impressive database.