"He's a good chap in his way. Serious. Not one of those noisy bastards at the Continental. A quiet American," says Thomas Fowler, describing Alden Pyle to the inspector, Vigot. "A very quiet American," Vigot concurs.
Fowler and Vigot are referring to Pyle's soft-spoken nature, but also to his secret career. Ostensibly employed to bring economic aid to Indochina, Pyle is in truth an intelligence officer, most likely CIA, who's labored behind the scenes to empower a Vietnamese general who's gone his own way.
Pyle is also dead at this point, having been killed. His murderers will not be found. The truth will not come out. Fowler knows what happened to Pyle, but he's mum to everybody but us.