HARTFORD, Conn. — A reputed mobster told an undercover FBI agent that he had access to two of the long-sought paintings stolen from Boston’s Isabella Stewart Gardner Museum in 1990 and could negotiate a sale of each for $500,000, a prosecutor said Monday. Robert Gentile, a 79-year-old convict from the Hartford, Connecticut, suburbs with a criminal record dating to the 1950s, allegedly made the offer following his release from prison a year ago, prosecutor John Durham said. The subject of the biggest art heist in U.S.