I have a feeling it was a domestic terrorist group behind the MA 370 flight...
I suppose, I will also mindlessly speculate:
Malaysia is a country with some deepening ethnic tensions between the Malay majority and the minority ethnic Chinese and Indian communities. In recent years both groups have been persecuted by the majority and have been blocked from various jobs and civil service positions. A majority of the passengers on board were of Chinese descent.
The Prime Minister's party is part of the Malay majority and has radical elements within the party. I think the lack of transparency and reluctance in their information sharing is due to the fear of some political fallout due to possible connections between the hijacker(s) and politicians. Corruption is also rampant in the country. How else could fake passports be used so easily? And why were those two Iranians connection dismissed so easily by the Malaysian officials? Now, I know they were probably "asylum seekers", but they used fake passports. It raises red flags right there and dismissing it so flippantly was strange.
Within a week we should have known much more about the passengers, the crew members, and the pilots themselves. All I know is that the pilots invited attractive women into their cockpit on other flights and smoked with them - all of which is very suspicious behavior. But besides that, I've heard nothing about them.
I believe by this point after 9/11 we knew much, much more (granted the circumstances weren't particularly mysterious as this).