Question: How did the FBI agent know that the two FBI agents on the plane were killed earlier?
Question: When Cutter says, "Have you ever played roulette?", how did Cutter know Charles' and Vincent's names? On the phone with Charles, he mentions both names.
Answer: I just rewatched it this morning. When Rane is trying to persuade Sheriff Biggs that he means business, one body (presumably The Captain) is thrown out of the plane. Just before the passengers are released, we see a quick shot of yo ambulances. One has 3 full occupied body bags in it. Therefore, it's implied the other two are in the other ambulance. This takes place before the FBI guy gets there. The fact that he tells Rane, those guys you killed were my friends" means that at some point they were identified by name. He could've found out numerous ways (Washington told him, briefing, the paramedics, Biggs, etc).
Answer: Everyone knows Charles Bane as the world's most dangerous terrorist. Plus, flight attendant Marti kept him informed of what was happening and who was involved. When they went below, Vincent introduced himself and explained he was not one of the terrorists.
Question: How can the plane take off from this country airport when they seem so worried about a short landing?
Answer: The plane would not be able to take off from that small airport.
Answer: It's been a while since I've watched this movie, but he obviously knows the plane is hijacked. It's a pretty safe assumption that the agents are dead as they're escorting a dangerous, high profile prisoner. Pretty easy to connect the dots. Alternatively, they could've failed to check in, and he assumed the worst.