The case began when Mr. Antonini, 46, returned to his home in Key Biscayne, Fla. Within days, he began cooperating with American investigators, offering them insight into what they contend was an operation in which wealthy Venezuelan businessmen in south Florida plotted with officials as high up as Mr. Chávez’s spymaster and former vice president.

Image President Hugo Chávez of Venezuela with Cristina Fernández de Kirchner, now president of Argentina, and the former president, Nestor Kirchner. Credit... Ian Salas/European Pressphoto Agency

Mr. Antonini may have asked for American help after Argentine investigators sought his extradition. Two days after being caught with the cash, Mr. Antonini was seen in the presidential palace in Buenos Aires celebrating the signing of business deals with Venezuela, according to Victoria Bereziuk, an Argentine secretary who was at the palace and who was one of the passengers in the private plane with Mr. Antonini, according to Argentine investigators.

Alberto Fernández, Mrs. Kirchner’s chief of staff, later denied her assertion, saying the Argentine government had no record of Mr. Antonini being at the palace.

What is clear is that soon after leaving Argentina he agreed to be recorded, photographed and videotaped by F.B.I. agents as he talked to a number of Venezuelans in Florida, who coaxed, cajoled and outright threatened him to keep quiet and accept falsified documents about the origin and intent of the money, American investigators contend. The government made 41 audio recordings and eight videotapes, documents in the case show.

The surveillance took place in a series of restaurants and cafes. In one meeting, on Aug. 23 at Jackson’s Steakhouse in Fort Lauderdale, Fla., Carlos Kauffmann, 36, a Venezuelan businessman, told Mr. Antonini it was not in Venezuela’s best interest for him to have any “problems” in Venezuela. A Venezuelan lawyer assured Mr. Antonini that Venezuela’s national oil company, Petróleos de Venezuela, would pay for all legal expenses and financial penalties that might result from the suitcase scandal.

But by Sept. 7, when Mr. Antonini met with Franklin Duran, 40, a Venezuelan oil products tycoon and close friend, at a Quarterdeck restaurant in Fort Lauderdale, Mr. Duran was telling him that Venezuelan authorities no longer trusted Mr. Antonini and suspected that he was talking to American law enforcement.

At one point, two of the suspects, according to prosecutors, offered Mr. Antonini $2 million in hush money. On another occasion, prosecutors said, Mr. Duran told Mr. Antonini that if he did not cooperate his children could be in danger.