Federal prosecutors have formally charged a fugitive in the 2015 highway robbery of 10 gold bars worth almost $5 million.

U.S. Attorney's Office in Miami announced Thursday that Pedro Santamaria, 56, has been indicted on robbery and gun charges. The FBI is offering up to $10,000 for information leading to his arrest and conviction.

Santamaria, Adalberto Perez and Roberto Cabrera caused noxious fumes to fill the cabin of the TransValue Inc. truck that was transporting gold bars from Miami to Boston, prosecutors said. Armed robbers removed and bound the TransValue employees, transferred the gold bars to their vehicle and fled.

Nine of the 10 bars were melted down for jewelry, Perez and Cabrera were sentenced to long prison terms, and others pleaded guilty after helping investigators. But most of the cash value of the stolen gold, which weighed about 275 pounds, has never been recovered, and Santamaria's whereabouts remain a mystery.

The prosecutors' statement doesn't say what prompted them to obtain an indictment now for Santamaria, an ex-convict who has been suspected in the heist since 2015.

The Miami Herald reports that Santamaria was given up by one of his accomplices and a prison acquaintance who tried to fence one of the 10 gold bars in Miami.

The shipment of gold, which was refined by Republic Metals, was insured. It was being transferred from South Florida to another major gold company, Metalor, in North Attleboro, Massachusetts.

According to court records, the robbers placed a GPS device under the tractor-trailer to track the shipment. They also placed pepper spray in the truck and remotely activated it so the two courier guards would get sick and have to pull over on Interstate 95 in North Carolina.

At that point, three armed robbers in a white minivan confronted them, yelling 'Policia!" Records show the guards got out of the truck without their weapons. They were tied up and led into the woods. The men took the buckets containing the gold bars, along with 40 silver-stamped coins, and divided up the loot.

Santamaria reached out to a man he had spent time with in prison to fence one of the gold bars, according to an FBI affidavit filed with Santamaria's criminal complaint. That man, Miguel Bover, approached a Miami pawnshop broker and asked him to sell the bar to a precious metals refinery. The plan backfired when an off-duty police officer working at the refinery grew suspicious and contacted the FBI, which confiscated the bar.

Bover was arrested in 2015, and eventually cut a deal and was sentenced to prison. He provided insider information on Santamaria and the others.

The Herald reports that jeweler Yanan Mirabal, who received a probationary sentence for initially lying to the FBI, estimated he made about $1.2 million by melting down and selling off the other nine bars.

Perez and Cabrera were arrested in 2016. Agents found more than $600,000 in cash, but no gold. Cabrera was sentenced to nearly 20 years and Perez received more than 11 years.