Complex sentence: A vicious prison guard is killed and the warden and some of his men held as hostages, but the National Guard have been sent for and block the escape with machine guns, gas and hand grenades.
Split into simpler sentences: A vicious prison guard is killed and the warden and some of his men are captured, as hostages. But the National Guard have been sent for and attack the escaping prisoners with machine guns, gas and hand grenades.