How do you blow up the bridge in socom us navy seals

In Fireteam Bravo 2, you might be in the level when you're in the water and there's 2 guards on the bridge. To blow the bridge, point your reticle at your target and on top of your reticle should say PLANT SATCHEL or PLACE C4. What I did is that I told my partner to hold his position, and quietly, I sneak beside the bridge and plant the satchel charge and then I try to not get detected while moving away from the satchel but ALWAYS get detected and when the bridge blows up, the 2 guards die. But sometimes, they quickly move to the other side of the bridge to avoid death.