Go inside the mansion and into the ballroom. Find the
tall agent that is by himself next to the piano. Wait until the
Ambassador (as shown in the photo) enters the room to talk to
the agent about the briefcase inside the safe. After they are
done talking, they will exit the ballroom and the Ambassador will
escort the agent to the room with the safe. Follow them, but do
not get too close. They will enter the room with the safe and
will talk to each other before the Ambassador opens it. Go inside
the room next to the room that they are in without looking suspicious.
Look inside the keyhole in the room that you are in, and you can
see the agent pointing a .54 pistol at the Ambassador to kill
him. Open the door and wait until the Ambassador opens the safe.
This is your chance to take out the agent silently. Take the briefcase
inside the safe and exit the room. The Ambassador will be directly
in front of the table, very scared.