How do you kill with a knife on SOCOM US Navy SEALs Fire Team Bravo

I don't know how to a knife kill in Fireteam Bravo 1, but for Fireteam Bravo 2, sneak up behind him/her and on top of your reticle will have a square button and the word beside it is "Knife". The goal is to maintain stealth. You can only do the knife kill when you are behind the enemy. For Fireteam Bravo 3, you cannot do a knife kill. You can only do something called a Stealth Kill. Sometimes stealth kills include knife. If you do a stealth kill, you might first choke him from behind and then stab him with a knife 3 times. The goal is to maintain stealth.