The announcement came nearly a month after the finance ministry’s Financial Intelligence Unit revealed that Peña Nieto was facing a probe over more than a million dollars of international money transfers.
The ex-president, who was in office from 2012-2018 and now lives in Madrid, has denied any wrongdoing.
Some of the allegations involve Spanish construction company OHL, the Attorney General’s Office said in a statement, without giving details.
“Progress in this investigation will allow prosecutions in the coming months,” it said.
Pablo Gomez, head of the Financial Intelligence Unit, told reporters in July that Peña Nieto had received around 26 million pesos ($1.25 million) sent by a relative in Mexico.
The movements had come under scrutiny because they were cash transfers and the origin of the funds was unknown, he said.
Peña Nieto is also believed to have ties with two companies that won lucrative contracts with the Mexican state during his term, Gomez added.
Peña Nieto tweeted in response that he was “certain that I will be allowed to clarify before the competent authorities any questions about my assets and demonstrate their legality.”
Mexican President Andres Manuel Lopez Obrador, a left-wing populist who replaced Peña Nieto in 2018, has repeatedly denounced alleged corruption under his predecessors.
A referendum Lopez Obrador championed last year on whether former presidents should be prosecuted failed to draw anywhere near enough voters to the polls for the exercise to be binding.