Northrop Mystery [The Bolt]: An Almost True Story

The year is 1958, and during a party, Brett Northrop, rich, charismatic owner of the Northrop Iron Foundry is found dead in his study, the door of which is bolted on the inside. The window, too, is locked; with no other apparent way, in or out.
Suicide, by all accounts; BUT IS IT? Brett Northrop has made many enemies, and any one of them would like to see him dead. But who killed Brett and how? A further murder and an attempted murder add to the intrigue.