Yeah it was definitely a thermobaric explosion as the vacuum effect made that clearly obvious. That helps narrow things down significantly as that is pretty high end stuff and not something that is usually in the hands of terrorists.
1) For a deployment of that particular weapon you can either altitude, ship deploy, hand deploy through launcher, manual set up, or on site robot deploy.
2) We saw fireworks going off before the explosion. This indicates a fire in the building beforehand. In fact we saw there was a fire going off beforehand. The fire was likely the trigger given what we saw so that means it was likely a device in place.
3) Given the seemingly putrid levels of security on that port (could be wrong but just what it looks like to me) it could have either been setup after a docking, through a dockhand, through an aquatic drop operation, through many, many different methods.
4) The fire is the key. When was the fire started, who started it, where was the fire origination point, why was the fire not immediately contained, and for how long did the fire burn for. Whoever started that fire is likely the same person/group who setup the device as they used the fire as a type of wick while they were able to get to safe distance if this was the method. Meaning the fire likely was started at enough distance for them to get back into the water and reach safe distance.
5) Who came into port today? One of those ships likely carried both the device and group/person who did it as the ship would be a much easier method of transport with much less risk of discovery.
We also need to look into:
a) Were any diplomats/anyone special in Beirut today? I guarantee someone was, just need to figure out who.
b) What have been the responses from the potential antagonists. They always tell us a lot about things by how they offer their statement.