BUENOS AIRES — Law-enforcement officials in Argentina suspected they would find plenty of weapons when they began raiding houses across the country this week as part of a monthslong investigation into the thriving arms trade that is outfitting Brazil’s violent drug trafficking gangs.

It soon became clear that this was no ordinary gun bust.

Investigators found anti-tank land mines, artillery rounds and hundreds of rifles in a network of houses, some outfitted with secret bunkers where American-made weapons were assembled.

Officials on Friday called it the largest weapons seizure in the country’s history as they disclosed details of a transnational investigation sparked by a couple of packages that caught the attention of customs officers in Miami last October.

More than 2,500 firearms were recovered. Officials said some of the arms came from Germany, the Netherlands and Spain. They said they had not yet established how many of the weapons were made in the United States.