Hide in Plain Sight -- 1980 (James Caan) true story

In 1960's Buffalo, New York, divorced Thomas Hacklin, Jr. is an honest, hard working working class stiff who adores his two young children, Andy and Junie, more than anything in the world. His ex-wife Ruthie has custody of the children, but Tom has all the access he wants to them. Tom doesn't like Ruthie's new boyfriend, Jack Scolese, and Jack spending time with his children, but he isn't quite sure what it is about Jack he doesn't like. Indeed, Jack is a low level mobster trying to make his way up, which he attempts to do by masterminding a major daytime bank robbery. It doesn't go quite the way Jack envisions as his mob bosses provide him no support in the matter. So Tom is not happy when he learns of the robbery, Jack's arrest and the fact that Ruthie has married him. Sam Marzetta of the Buffalo PD and Jason Reid of the Justice Department convince Jack to testify against his mob bosses in return for protection for him and his new family in the witness relocation program. So one day when Tom routinely goes to pick up his children from Ruthie's, he finds that all three have vanished without a trace. He eventually does learn that the authorities have in a sense taken away his children. With the help and advice from his new girlfriend and eventual second wife Ali, Tom uses whatever means possible - including legal and political - to find his children. The police won't even talk to him until he starts legal proceedings to sue them and the government. But when these proper channels don't seem to be getting him anywhere, he decides he needs to take matters into his own hands, which isn't easy as the authorities do whatever they can to hide Jack, Ruthie and kids from Tom, while the mob does whatever it can, including using Tom, to find Jack.