Complex sentence: A few hours after the hijacking had commenced, a note was received at the Lufthansa headquarters in Cologne: the aircraft would be blown up by 09:00 on the following day if a ransom of $5 million had not been paid by then.
Split into simpler sentences: A few hours after the hijacking had commenced, a note was received at the Lufthansa headquarters in Cologne. The aircraft would be blown up by 09:00 on the following day in case a ransom of $5 million had not been paid by then.