Everyone who enters the US is photographed and fingerprinted (e.g. tourists, visitors, etc).

Everyone who applies for a US visa is additionally photographed and fingerprinted.

Everyone who applies for naturalisation is additionally photographed and fingerprinted.

The suspects did all three of these things. The US government has almost every foreigner legally on American soil in a database.

Plus then you have the DNV on top of that and or passport office.

The older brother had been questioned by the FBI before.

http://www.cbsnews.com/8301-201_162-57580534/fbi-interviewed...

He's 19, so he likely has a driver's license or other state-issued ID photo on file.

Best random guess: Something related to him being an alien and becoming a naturalized citizen

I'm going to venture to guess that either a) the officer was talking out of his ass to make a quick point or b) someone, somewhere massively scraped every social network they could think of for anyone with ties to the area or the event. That being said, I don't really like the sound of it either, regardless of how true it is.

--pre-submit thought: perhaps it was the older brother? Our background is rather limited but it would not be surprising if he had had prior run-ins with the law. Even a minor offense will get your mugshot in the file.