Let A be a treasure thief, then A tells a lie, B tells a lie (treasure thief is A, not D)C tells a lie (treasure thief is A, not B)D tells the truth, then A is treasure thief D tells the truth.

Suppose B is a treasure thief, then A's words can't be sure whether they are true or false, C is telling the truth, and D is telling the truth, which contradicts the topic.

Let C be a treasure thief, then A's words can't be sure whether they are true or false (they may be at the scene but didn't steal the treasure), B and C tell lies, and D tells the truth.

Suppose D is a thief, then A can't be sure whether it is true or not (it may be at the scene but it didn't steal the treasure), B tells the truth, and C and D tell lies.

Only one of them is telling the truth!

So A is a treasure thief D, telling the truth.