That was his father.
3 seconds sound clip from the Cruising movie soundboard.
You can hear this line at 01:32:41 in the Blu-ray version of the movie.
Quote context
[...]
- Burns... Welcome to the Detective Division.
- Thanks.
- Do you know any of these people, Mr. Gaines?
- Er... they look like pictures of Stuart when he was a kid.
- I got a box of old letters here. All addressed to the same guy: John L. Richards, St. Louis.
- I wonder why he never sent these.
- What time did you hear the voices?
- Did you hear anything they were saying?
[...]