Guards and police inspect a vault that dozens of attackers blew up in Ciudad del Este, Paraguay, on Monday. (AP Photo/Mariana Ladaga/Diario ABC Color)

MEXICO CITY -- The commando-style raid in the early hours of Monday unfolded with military might and coordination.

Dozens of gunmen blocked highways, burned trucks and cars, sealing off the perimeter of their target for hundreds of yards. The assailants, who were wearing flak jackets and driving in armored vehicles, used explosives and .50 caliber guns to blow the facade off a transportation company office in Ciudad del Este, a town in Paraguay near the smugglers' haven in the border region with Brazil and Argentina. They killed a policeman, broke open the vault and then escaped -- apparently fleeing by motorboats up the Paraná River -- with millions of dollars.

The haul was initially estimated at $40 million, which would be roughly equivalent in today's dollars to the amount lifted during the "Great Train Robbery," the infamous 1963 theft of a post office train in England. Officials with the targeted company, Prosegur, which transports valuables, said the amount was less. Still, Paraguay news reports were calling the robbery the largest in the country's history and the "heist of the century."

Paraguayan authorities said the gunmen were Brazilians, and news reports suggested this was possibly the work of a Brazilian criminal organization, First Capital Command, known by its Portuguese initials, P.C.C. The gang, which began in the prisons of Sao Paulo in southeastern Brazil, was also involved in the deadly prison riots in Brazil earlier this year.

Lorenzo Lezcano, one of Paraguay's top security officials, estimated that 40 to 50 gunmen took part in the heist and that it was "well planned."

After the robbery, Brazilian police tracked some of the alleged assailants close to Foz de Iguazu, Brazil. A shootout there left at least three gunmen dead. Four others have been arrested, according to Paraguayan officials. The Paraguayan government tweeted out photos of some of the slain gunmen lying in the grass, wearing what appeared to be camouflage.

The border region of Paraguay, Argentina and Brazil, known as the Triple Frontier, has long been a hub of illicit activity, particularly smuggling. U.S. and South American authorities in the past have suspected that the area has even been used by Hezbollah operatives and other possible Islamic extremists and underworld figures from the Middle East.

There have been other similar military-style assaults on companies that ship valuables in Brazil and Bolivia in recent months that authorities suspect might be connected to the same band of criminals. The Bolivian interior minister, Carlos Romero, asked for high-level meetings with Brazilian and Paraguayan authorities to discuss the recent crimes.

After the assault, the Argentine government announced that it was going to take additional security measures on its northeastern border in case some of the gunmen head that way.