Here's a breakdown:
* Initially: He suspects the other guests, especially the seemingly fragile Vera Claythorne, of being the murderer, especially as she has a strong motive.
* Later: He starts to consider the possibility of the mysterious Mr. Owen being involved, especially after discovering the poisoned chocolate box.
* Towards the end: He realizes that the killer could be someone else entirely, someone they never expected. This leads him to believe that the killer might be the one manipulating them all, playing a twisted game.
However, it's important to remember that Lombard is not the narrator, so we only get his suspicions through his actions and dialogue. He's never entirely sure who the killer is until the very end.
The identity of the murderer in "And Then There Were None" is a significant plot twist, and it's best discovered by reading the book.