Meir Javedanfar speculates about the potential actors and motivations behind the plot:
It could mean that Iranian Supreme Leader Ayatollah Ali Khamenei, the ultimate authority for permitting attacks of this nature, is willing to go further than simply hurting US and Saudi interests through proxies in Iraq and Lebanon. This would set a new precedent, as Iran previously shied from launching operations against US and Saudi interests on their own soil. Yet there’s another, somewhat intriguing possibility. Could elements within the Iranian government or security establishment have planned this attack, without Khamenei's knowledge in order to hurt him and his regime? On the surface, at least, this seems unlikely. However, details of the US claims certainly suggest that the idea isn’t completely without merit.
Doug Mataconis collects and assesses some other options.