Originally Posted by
flyerCO
Also some countries can require that X Percentage/number of passengers be randomly selected. This accounts for a large number of SSSS on the flight from that country, but PRECHECK on the connecting flights. TSA decides if you get Pre. However the foreign government can still require you be a selectee. I get this flying through LHR with VS/DL periodically. Even now that both are PreCheck airlines.
At LHR for VS/DL flights, it's not the UK requiring this. For those flights from the UK to the US, it's the US Government's doing. So if this situation is one that hits and you want those responsible for it to look into a complaint about this: contact the US Government and the operating carrier, as the UK isn't in the prime positions to address a passenger complaint about this on flights from the UK to the US on DL/VS.