The best explanation for the reason and 'who dropped the bomb' is a power hungry government official.

The official is the Secretary of State. He got bombs from either the US's arsenal or from a foreign government to be framed.

He detonated the bombs while the President was giving the speech because everyone who is ahead of him on the President succession ladder would be attending the speech.

Since he is the diplomat for the country he could easily been overseas during the explosion.

He organized a terrorist to get potential bombers, but not tell them what they were transporting to the target cities.

A group of the potential terrorists, Hawkins included, found out about the plot and decided not to say anything because it was too late, or it would have panic the country and bring clogged highways that could also be targets.

Because of the dangers, Hawkins and his conspirators decided to find towns that would not be affected by the attacks and move their families there.

His follow deserters were listed on the computer in the "Trap House."

The reason for the hunter, who is working for the Secretary of State, is to hunt down and kill the deserters to the attack. Sarah was on board with Hawkins's desertion plan, or so he thought.

Sarah was actually working for the hunter who was killing the deserters. The man that Hawkins knew with the refugees from Denver was a fellow deserter.