I was the thief ??

No one but him could have told them that the thief was I

除了他之外，沒有人能夠告訴他們賊就是我。

No one but him means "nobody other than himself", or "he is the only one".

Other than himself, nobody else could have told them that the thief was I.

2009-12-17 16:20:33 補充：
當然，這句可寫成：
He is the only one who could have told them that I was the thief.

2009-12-18 06:13:04 補充：
同意hkman1357999的見解。"could have" 是可以有假設的含義的。在這句的意思上，可以解作："有可能會"。全句是： "除了他之外，沒有人有可能會告訴他們賊就是我"。整體上，和 "除了他之外，沒有人能夠告訴他們賊就是我" 的意思也相近。

A note to friends:

could have been 的中文譯可考慮加入"原本可以"，因當中有假設的意思．　

2009-12-17 22:42:24 補充：
修正：could have been ＝ "原本可以是"

could have told = "原本可以告訴"

2009-12-18 07:23:31 補充：
參考中譯:

可以告訴他們賊就是我的人, 原本只有他。 (但他結果沒有這樣做)