Hillsborough County Sheriff Chad Chronister tells TMZ, he's convinced one person could NOT have committed the 1997 murder ... he says someone must have helped and someone got paid.

Although Hillsborough says Don's wife, Carole Baskin, is NOT a suspect or person of interest, he tells us he's suspicious of her and he's also suspicious of others. Watch FOX Monday night at 9 PM for our special, "TMZ Investigates, Tiger King: What Really Went Down?" He explains exactly why he's suspicious.

The Sheriff's looking for someone to come forward -- maybe someone who helped who wants to clear his or her conscience -- and spill the beans.

Sheriff Chronister also makes it clear ... he's willing to deal with anyone who has info, and immunity is on the table.

Carole has strongly maintained she had nothing to do with Don's disappearance.