The CP 1921 mission is one of the main objectives of the game. You have to find out what happened at the last briefing and get back to the control room before the terrorists manage to get their hands on the nuke. There are various paths you can take like going left or right, through doorways or using a sniper rifle. Actually you have several ways to complete this mission. However, I would like to point out some key points that might help you get through it easier.
Firstly, look out for the clues and items that will let you know where the objective is located. For example - if you shoot at the black box where there is a dead body, the next screen will tell you where it is. Use binoculars as you go through the environment and look out for anything that looks suspicious or out of place. Keep your eyes open for the doors that lead to the next part of the level, otherwise you might miss an objective and end up failing the mission. You might also want to watch out when there are people walking past. If you're too close, you'll miss the objective.