Complex sentence: O'Neill ascertained that Hanssen was using a Palm III PDA to store his information; when he was able to obtain Hanssen's PDA briefly and have agents download and decode its encrypted contents, the FBI had its "smoking gun".
Split into simpler sentences: O'Neill ascertained that Hanssen was using a Palm III PDA to store his information. When O'Neill was able to obtain Hanssen's PDA briefly and have agents download and decode its encrypted contents, the FBI had its "smoking gun".