Complex sentence: She was also seen around town with gangster Jack "Legs" Diamond, with whom Crater was rumored to socialize', he had known organised crime overlord Arnold Rothstein and been extremely upset at his murder.
Split into simpler sentences: She was also seen around town with gangster Jack "Legs" Diamond, with whom Crater was rumored to socialize'. He had known organised crime overlord Arnold Rothstein and been extremely upset at his murder.