Complex sentence: According to the officials, the hijacker entered the plane's cockpit while the plane was in Greek airspace and the plane sent the hijack code twice at 14:55 UTC, while it was cruising about 25 km north of Thessaloniki.
Split into simpler sentences: According to the officials, the hijacker entered the plane's cockpit while the plane was in Greek airspace. The pilots transmitted the hijack sqawk code twice at 14:55 UTC, while it was cruising about 25 km north of Thessaloniki.