He drilled through the coalface... into another seam run by the NSA.
It sounds like he drilled through the coalface into the NSA's working coalface, (time is money as they say) met eye to eye and then tried to head back to his own but they said "Hold on there a minute - Son".
In this line of work, there is a slither/seam of rock between legal and illegal here, in terms of coalface he was working*. Like Potholing, its easy to get over enthusiastic, look back and see a very dim light and wonder how you'll get out with the water rising around you.
I think he forgot too, that American still arrest and ask questions later, just based on the colour of someone's skin, especially (it seems) in the post Trump era.
I'd like to think we were a bit more tolerant in the UK, but given we've AGAIN chosen Notting HIll over the Chelsea Flower show to trial intrusive optical facial recognition cameras this year (which is then cross-referenced to Immigration/Border Control data), it's pretty obvious that discriminatory racial profiling is alive and well in the UK too.
Trialling Facial Recognition software? I mean a face is a face right? Black or White - right?. The Chelsea Flower Shows seems to be the place it should have been trialed this year, if Notting Hill was the trial location last year.
Unless of course, it has nothing to do with trialing this technology at all, more a case of "using it", with no legal/privacy constraints, under the guise of auspicious circumstances.
This sort of stereotyping really doesn't encourage any bright individual (especially from a multi-racial background) to work for GCHQ/NSA, when Security Services act like this.
These two organisations are joined at the hip, to say GCHQ didn't know of his impending arrest has to be utter lies. They let him go to the States, knowing full well he'd be arrested on this return. There is a total disregard showing here with regard to the intrinsic safety, towards UK Citizens by the UK Government.
My worry is too, Iain Thomson might need to be more careful in future (has posted some very good past articles on this, with links to malware samples). It's not a good time to be a Journalist either, travelling between US and UK. Take care Iain.
*(He should have taken some tips from "Telfon" Tony Blair). (he who can do no wrong in terms of the establishment)