Thief

Frank (James Caan) is a highly experienced jewel thief and hardened ex-convict who has a set structure to his life. With a pair of successful Chicago businesses (a bar and a car dealership) as fronts for his lucrative criminal enterprise, Frank sets out to fulfill the missing part of his life vision: a family beginning with Jessie (Tuesday Weld), a pretty cashier he has begun dating.After taking down a major diamond score, Frank gives the diamonds to his fence, Joe Gags (Hal Frank). However, before Frank can collect his $185,000 share of the score, Gags is murdered by being pushed out of a 12th-story window for skimming off mob collection money. Barry (James Belushi), Frank's friend and associate making the pick-up, discovers that a shady plating company executive Gags was working for, Mr. Attaglia (Tom Signorelli), is responsible for Gags' murder and stealing Frank's payoff. In a tense confrontation at Attaglia's plating company, Frank demands his money back.This leads to a face-to-face meeting with Attaglia's employer, Leo (Robert Prosky), a high-level fence and Chicago Outfit boss. Unknown to Frank, Leo has been downing Frank's "merch" from Gags for some time. He admires Frank's eye for quality fenced goods and professionalism, and wants him taking down contract scores all over the country working directly for him, offering Frank large profits. "I'll make you a millionaire in four months," Leo states at their jargon-filled meeting, which is monitored from a distance by police surveillance -- as well as a hidden Barry armed with a rifle, in case the meeting goes bad.At first, Frank is reluctant to consider Leo's offer, not wanting to "deal with egos" or the added exposure. But later that night at a coffee shop, an emotional bonding with Jessie changes the game when she agrees to be part of his life after he relates a harrowing tale of prison survival by way of a toughened mental attitude. To hasten his plans, Frank now agrees to do just one big score for Leo, telling Barry that this will be their last job. With a little help from the paternal Leo after being rejected at the state adoption agency, Frank is even able to acquire a baby on the black market, a son he names David after his recently deceased mentor and closest friend from prison, nicknamed Okla (Willie Nelson).Things start heating up on account of Frank's new association with Leo. After resisting a shakedown from a group of corrupt police detectives, then subsequently ditching their surveillance, Frank and his crew are soon involved in a large-scale West Coast diamond heist organized by Leo. All goes well with Frank's "burn job" and he is expecting the agreed-upon sum of $830,000 on $4 million wholesale of unmounted stones. But when Frank returns from the job, Leo tosses him an envelope containing less than $100,000, the "cash part" according to Leo, who says he has invested the rest of Frank's cut in shopping centers, an idea Frank had flatly rejected. In addition, Leo sets up a Palm Beach score for Frank in six weeks without consulting him. An irate Frank bluntly tells Leo that their deal is over, takes the envelope of cash as he leaves and demands the rest of his money in 24 hours or "...you will wear your ass for a hat."Frank drives to his car lot unaware that Leo's henchmen have already beaten and captured Barry, and are waiting in ambush for him. In the set-up, Frank is knocked out with the butt of a carbine and Barry is killed by several shotgun blasts from Carl (Dennis Farina), one of Leo's main enforcers. Frank awakens with Leo staring down at him, surrounded by his henchmen. Leo coldly informs him that he, Jessie, their child, and everything he owns are Leo's property. He threatens to prostitute Jessie and "...whack out your whole family" if Frank does not continue working for him. Leo warns Frank to "tighten up" and focus on his responsibilities; meanwhile, his henchmen dispose of Barry's body.Frank goes home to contemplate his next move, gradually sinking into a hardened apathy akin to his prison days. He coldly orders an uncomprehending Jessie out of their house, telling her their marriage is over. He instructs an associate to drive her, the baby and $410,000 in cash somewhere where they cannot be found, informing Jessie more money will be coming at regular intervals, but that he will not be joining her.With nothing to lose, Frank blows up their home in a fiery nighttime blast using high explosive charges. He then drives to his business establishments and does the same, destroying his bar in a violent explosion then setting fire to his car lot. Armed with a pistol, he quietly breaks into Leo's house in a peaceful neighborhood and pistol whips Attaglia in the kitchen. Frank hunts for Leo, who is hiding, armed with a .357 Magnum revolver. Leo is inevitably located and gunned down. Frank then pursues Attaglia as he tries to escape from the house, but is promptly confronted in the front yard by Carl and another henchman. In the ensuing gunfight, Frank is shot, but manages to kill the trio. Frank loosens what appears to be a ballistic vest he was wearing beneath his jacket, and walks away into the night.(Copied from Wikipedia)

Question: Things start heating up because of Franks new association with who?
Answer: Leo