A man convicted years ago in St. Paul for killing his infant son was sentenced this week in Florida for stealing a centuries-old gold bar worth $550,000 from a Florida museum.

Richard S. Johnson, 41, was given a federal prison term of more than five years in connection with the 2010 theft of the gold antiquity from the Mel Fisher Maritime Museum in Key West.

The 4⅔-pound breadstick-shaped bar had been secured in a clear case with an opening that allowed millions of visitors to touch it. It had been recovered from the Santa Margarita shipwreck by Mel Fisher and his crew in 1980. The Santa Margarita and a second Spanish galleon were among eight vessels that sank in 1622 during a hurricane.

The 16.5-karat gold bar had been on display for more than 20 years until Johnson broke into the case and put the bar in his jeans. Authorities said they recovered only a sliver of the bar, with the rest "callously hacked … to pieces" and sold for scrap and likely never to be recovered, according to prosecutors.

Insurance covered less than one-fifth of the bar's value. Johnson and a co-conspirator were ordered to make full restitution for the theft, but their ability to meet that obligation is in doubt.

In January, an anonymous tip led to Johnson's arrest in the Sacramento area where he was living. He was charged with breaking into the bullet-resistant case after the museum closed, while co-defendant Jarred A. Goldman, 32, of Palm Beach Gardens, Fla., stood watch outside. Goldman was sentenced to more than three years in prison.

Before sentencing, Johnson argued in a court filing for a two-year prison term. He asserted that he remained law-abiding from the time of the theft until his arrest more than seven years later and expressed his "extreme remorse."

He also pinned some of the blame for his crime on his $700-a-week marijuana habit and being abused as a child.

In 1997, Johnson pleaded guilty to first-degree manslaughter for shaking his 6-month-old son to death in response to the baby's crying in the family's St. Paul home. He served the first 6¼ years of a 10-year term in prison and the balance on supervised release.

Paul Walsh • 612-673-4482