So says this report. The details is what makes the story convincing. Most analysts would agree that the Lebanese security forces, under heavy influence from Syria, would capture somebody and at least claimthey were behind the Mughniyah assassination, just to save face and point the finger at the Israelis.
Prior to this story, the most popular story was that Syria killed Mughniyah, as part of an attempt to reconcile with the West and remove one of the levers of control Iran had been using to threaten the Assad regime. However, Iran had many ways of influencing Syria besides Mughniyah, and the U.S. raid into Syrian territory puts to rest the theory that some deal had been secretly reached or was imminent.
As most originally suspected, it does appear that the Israelis were behind this very impressive operation.