The plan involved at least two Mexicans, a Canadian and a Dane, the Interior Secretary, Alejandro Poire Romero, said.
Several arrests were made in Mexico City last month, and four suspects are being held.
Saadi - the third of Gaddafi's eight children - never made it across the Atlantic. He fled his country in September for neighbouring Niger, which granted him asylum on "humanitarian" grounds.
He is now under house arrest there as Interpol tries to extradite him.
Poire said Saadi paid fixers to open bank accounts in Mexico and buy at least one multimillion-dollar property.
"The activities of the criminal organisation included falsification of official documents, opening bank accounts with false documents and the purchase of real estate intended, among other things, to serve as a residence for the Gaddafi family," he said.
After a tip-off, local authorities began monitoring the group in September.
Its alleged leader is Cynthia Ann Vanier, a Canadian security consultant who worked with the Gaddafi regime.
Poire said she has been charged with falsifying papers and alleged she was "the direct contact with the Gaddafi family, leader of the group, and presumably in charge of the finances".
Also in custody are two business partners of Gregory Gillispie, an ex-United States Marine based in California, who supplied an aircraft that recently flew Vanier to Libya.
He told Canada's National Post newspaper that there was "no evidence of wrongdoing" against his colleagues.
Of Saadi's siblings, brother Muhammad and sister Aisha are thought to be in Algeria, brother Saif al-Arab was killed in an air strike and Saif al-Islam was captured last month.
A former professional footballer, Saadi captained Libya's national team and was briefly signed to the Italian club Perugia before failing a drugs test.
He later started a business career.
Interpol has requested his arrest on suspicion of helping his family to steal billions of dollars of Libyan state funds, and the United Nations has frozen his assets.
His lawyer, Nick Kaufman, said that despite refusing to surrender, he was "fully respecting the restraints placed on him by the international community".
- Independent