Speculation - most South Korean males must undergo military training due to the ongoing situation with North Korea... could be Cho was X - South Korean military?
Wikipedia :
Korea, South
South Korea has mandatory military service of 24 to 27 months.[8] There are no alternatives for conscientious objectors[9] except imprisonment.
There are many reported instances of American citizens of Korean descent being forced to serve in the South Korean military. Under South Korean law, one is considered a citizen if one’s name is entered into the Korean Family Census Register, or hojuk. At least two of these cases involved individuals whose names had been recorded on the Korean Family Census Register, without their knowledge. [11].
One of the witnessess in the classroom said that he changed clips on his gun quickly...I suspect either military experience or just familiar with guns in general...
Also, phone records show a listing for D Hui in Centreville, VA, with address and phone number. Could that be his father?
Yes.