Warning! A chance of completing this mission objective appears only when hostages are still alive, that is you were not late with flying to the plant. It doesn't mean of course that otherwise it is not worth to explore places where they were held, because in addition to finding their bodies you could also find ammo and receive experience points for getting through air vents.
Map legend: 1 - Entrance to the air vent leading to hostages.
Hostages are held in the room upstairs but I DEFINITELY do not recommend using the stairs, because an attempt to pass through the door will result in prematurely arming the bomb and starting the countdown. 20 seconds is a lot of time in theory but it is not worth to be stressed unnecessarily. Instead of that, stand near the place where you start. Use the right stairs and go down. Turn around - you'll find a crate #1. Go to the crate and move it to reveal an entrance to an air vent #2. If you didn't eliminate enemies described in Step 5 and you do not want to do so, you have to choose a perfect moment to enter the vent - no one can see you.
Go through the vent and after few moments you'll reach a ladder #1. Use it to go up. Enter another vent. After several meters you'll find yourself above Lab 1 #2.
Carefully go straight ahead until you reach another air vent #1. This time you will reach a room with hostages #2. If they are still alive, you'll find Greg Thorpe and some anonymous plant workers. If you're late for this mission, all hostages will be dead.
Approach the bomb mechanism (screen above) and interact with it. As you probably remember one of the terrorists left a message with instructions how to disarm the bomb. So instead of wasting time for mini hacker game just enter a correct combination - 1505.
Reward: 500 experience points.
Stay in this room, pick up tranquilizer riffle ammo and find Greg Thorpe #1. During a conversation you'll be asked to save his wife. Regardless of your reaction on his request #2, you'll unlock a new mission objective - Save Josie Thorpe (same one which would be active from the beginning if hostages are dead).