For two decades, a crew from Lackawanna County in Pennsylvania traveled to a dozen small museums across the country, where its ringleader smashed through security systems to strip them of art and precious memorabilia.

Their loot included treasured heirlooms from America’s past, including nine of Yogi Berra’s World Series rings, two of middleweight Tony Zale’s championship belts and 30 golf and horse racing trophies. Instead of selling the memorabilia, they melted much of it down and sold it as raw metals.

On Friday, three men involved with the theft ring were convicted in U.S. District Court in Scranton for conspiracy to commit theft of major artwork.

The convictions followed testimony from the former ringleader of the group, Thomas Trotta, who pleaded guilty and became a central witness at the trial of the men, who had been his friends since his days at North Pocono Middle School. According to federal prosecutors, Mr. Trotta broke into the museums, while the other men assisted him by either planning the robberies, melting down the stolen items or transporting them to be sold in New York City.

The three defendants, Nicholas Dombek, Joseph Atsus and Damien Boland were also convicted of concealment and disposal of major artwork and interstate transportation of stolen property. Their sentencing has not been scheduled.

A fourth defendant, Alfred Atsus, was acquitted on all charges, and his brother, Joseph, and Mr. Dombek, were acquitted on several of the counts put forward by the prosecution.