It's funny, the US Marshalls interviewed for this are extremely forthright in explaining their methods, but clam up and say they "can't explain these methods" as soon as they have any leads relating to cell phones. Probably because they're using the US's vast warrantless surveillance system to pull any possible info they can on her.
For example, they "track[ed] down the phone number for an American businessman they believed had connected with Armstrong at some point," and are cagey about how they got that number. I'd bet that they pulled her phone records and started cold calling everyone she's ever contacted through her cell phone until they got someone who could give them a lead.
Later, they set up the fake yoga instructor ad, and mention that they're tracking the phone location of the person who answered the ad to make sure they're at the sting location.
It's crazy that even with all those "methods the Marshalls won't go into," they almost gave up on finding her.
Fucking hell, this is such a callous response. In any other situation, the group representing the side that just had masked vigilantes attack peaceful demonstrators would make amends. "These people don't represent our movement. We disavow them and what they stand for." And so on.
I see they're taking a page from Israel's book: refuse to apologize, defend unprovoked violence, and blame the victims on top of everything else.