Anybody have any details on the so-called lockdown at IAD last afternoon (2 May)?

I got off UA975, entered the US, and while in line for scanning, TSA told us that nothing was taking off or landing in the whole airport, and nobody could get scanned until this was resolved. About 45 minutes, maybe an hour, after they stopped, they resumed scanning. As it happens, my flight did take off during the "lockdown", and the next one would have as well, except the pilots were in the main terminal and they weren't allowed to go to C/D.