Union Troops in Texas during Secession

[Shadow9216]

Question on the fate of Union soldiers in Texas during secession. I read that troops were told to abandon their western posts and march to San Antonio, where they expected to be evacuated. I don't know if this was before, during, or after the period when Twigg surrendered Federal troops in Texas, but I learned elements of the 8th Infantry were held as POWs until exchanged. I'd like to know:

1) What happened to Southern/Pro-CSA Federals? Were they allowed to return to their states, or were they held as well?
2) Where and for how long were the Federals held until the exchange?
3) When did this actually take place?

Internet searches have proven somewhat barren, so I'm hoping the experts here have some answers!