In cases of actual terrorists, the FBI almost certainly would not contact them by leaving a business card and asking them to call. The did it in this case because they already knew that he was almost certainly not a terrorist.
First, an actual terrorist would probably have left as soon as the guards came out, rather than argue with them over whether or not photography from public property is allowed. Staying and arguing gives the guards more time to note identifying details that could lead their plans being disrupted.
Second, since they tracked him down from his rental car, and since they FBI does in fact have basic competence, they knew by then that he was a professional photographer and everything about the incident was consistent with his expected behavior based on his profession and his publication history.
So why not drop it at that point? My guess is they just wanted to make sure it really was him who rented the car in his name. A smart terrorist would use a fake identity to rent the car, and what better than to use an identity of someone who you would expect to find out photographing things like their target?
Looking at the layout, he was probably on that little drive that parallels the freeway (that's where the nicer photo is), and it probably isn't a public street.
edit: Looking at Street View, he must have been somewhere on Victory, that drive is gated.
another edit: The aerials in 2004 probably would not have been quite so detailed. But they would still be pretty detailed.
First, an actual terrorist would probably have left as soon as the guards came out, rather than argue with them over whether or not photography from public property is allowed. Staying and arguing gives the guards more time to note identifying details that could lead their plans being disrupted.
Second, since they tracked him down from his rental car, and since they FBI does in fact have basic competence, they knew by then that he was a professional photographer and everything about the incident was consistent with his expected behavior based on his profession and his publication history.
So why not drop it at that point? My guess is they just wanted to make sure it really was him who rented the car in his name. A smart terrorist would use a fake identity to rent the car, and what better than to use an identity of someone who you would expect to find out photographing things like their target?