They assumed he was dead.
3 seconds sound clip from the Apocalypse Now movie soundboard.
You can hear this line at 01:50:36 in the Blu-ray version of the movie.
Quote context
[...]
- Jim, it's here. It really is here.
- There has been a new development regarding your mission which we must now communicate to you.
- Months ago, a man was ordered on a mission which was identical to yours.
- We have reason to believe that he is now operating with Colonel Kurtz.
- Saigon was carrying him MIA for his family's sake.
- Then they intercepted a letter he tried to send to his wife.
- Captain Richard Colby, he was with Kurtz.
- Disneyland. Fuck, man, this is better than Disneyland.
- Charles Miller Manson ordered the slaughter of all in the home anyway as a symbol of protest.
- That's really weird, ain't it?
[...]