First mission : Milwaukee Junction

In the first play-through, I completed the Typhoon objective first and found the hostages already dead when I arrived at the office where they were held. I figured that the gas bomb was activated when I was discovered by PF members. Also, I did not discover the overhead vent passage until much later. 

Today, in my second play-through, I reload the first save file (and skip the lab attack tutorial). I intended to save the hostages but I keep finding them gassed to death. Even using the overhead passage, I saw them dead through the window. I reload the mission 3 times and they ended up dead each time. 

At one time, while waiting in the clean room (as a cover for game level loading), I spot a subtitle text "Dispersal device activated. Have a nice day."

Giving up, I deleted the save file. I guess the only option left is to play from the start.

Did it happen to anyone else?