MEXICO CITY — The former head of Mexico's state-run oil company was extradited from Spain on Thursday for what could be a groundbreaking investigation into corruption within this nation's political elite.

Emilio Lozoya, the 45-year-old former chief executive of Pemex, has reportedly reached a deal with prosecutors to turn over evidence that politicians were bribed to support an energy reform championed by former president Enrique Peña Nieto.

While no one has been named, the legislation was negotiated by some of the country’s most prominent politicians. Lozoya, who is accused of accepting millions of dollars in bribes from Mexican and foreign companies, was also a key figure in Peña Nieto’s 2012 election campaign. That’s generated speculation that he could implicate the former president’s top aides — or even the ex-leader himself.

Peña Nieto has not been charged. He has denied doing anything illegal.

“It’s an unprecedented case,” said Sergio Aguayo, a political scientist at the College of Mexico. He said the investigation “is breaking with the tradition of impunity for presidents” that dates to the Mexican Revolution a century ago.

The case could strengthen the hand of President Andrés Manuel López Obrador heading into Mexico’s midterm elections next year. The longtime leftist won an overwhelming victory in 2018 in part on promises to battle endemic corruption. But he has made little headway on Mexico’s key problems — extreme violence and a lethargic economy — and has been criticized for failing to rein in a sprawling coronavirus outbreak.

Aguayo predicted that López Obrador would encourage an investigation of his predecessor, to claim a political victory in a difficult moment.

“What he can offer is the head of an ex-president,” he said.

Lozoya was arrested in Spain in February on Mexican corruption charges after months as a fugitive. He is accused in one case of receiving a $3.5 million bribe in connection with Pemex’s purchase of a run-down fertilizer plant in 2013. In a second case, he is charged with pocketing millions of dollars to steer contracts to Odebrecht, the Brazilian construction giant at the center of a massive regional kickback scandal. Part of that money was allegedly paid while he was the international coordinator of Peña Nieto’s campaign.

Peña Nieto has denied his campaign was financed illegally.

Odebrecht pleaded guilty in 2016 to U.S. charges that it had handed out hundreds of millions of dollars in bribes to companies on three continents, and agreed to pay at least $3.5 billion in fines. Investigations into its corruption schemes have ensnared scores of political leaders and business executives, upending the political systems in many Latin American countries.

The company admitted in the settlement to paying $10.5 million in bribes to unidentified Mexican officials. But judicial investigations into Odebrecht stalled under Peña Nieto.

Lozoya has long said he was innocent of corruption. In recent months, his lawyers have said the former official acted on orders from higher-ups, including the ex-president.

Lozoya agreed to be extradited from Spain as part of a deal in which he offered to provide at least a dozen videos of Mexican lawmakers receiving bribes to approve the 2013 energy reform, according to the newspaper Reforma. Peña Nieto had portrayed that law as a landmark effort to open the struggling state-run oil sector to greater foreign investment. López Obrador has strongly opposed it.

On Thursday, López Obrador said Lozoya’s extradition could lead to a change in how Mexicans perceived corruption, stigmatizing officials who enriched themselves but got off scot-free.

“This extradition will help quite a bit in purifying our public life, in cleaning up the corruption in our country,” he said. He spoke a few hours before a Mexican government jet departed from Madrid with Lozoya aboard.

Mexican officials have often used corruption investigations to target political rivals or enemies. Analysts said the Lozoya case could be a breakthrough in shedding light on longtime corruption — or it could be a cudgel used by López Obrador against his opponents.

“There are two possibilities,” the historian and novelist Héctor Aguilar Camín wrote in the daily Milenio. “One is an exemplary, unprecedented legal process, and another, an unprecedented use of the justice system for political ends.”

López Obrador, who took office in December 2018, has had a mixed record on corruption so far. He has pursued high-profile targets such as Lozoya, and has reopened the country’s most notorious case of forced disappearance, involving 43 students from the southern town of Ayotzinapa who vanished in September 2014. He’s regarded as personally honest and has dramatically scaled down the trappings of the presidency, jettisoning the armies of aides that once surrounded the Mexican leader and flying commercial.

But critics say he’s turned a blind eye to allegations of impropriety and questionable real estate deals involving members of his own cabinet.

“The government is not without contradictions,” Aguayo said. “But given what Mexico was, the serious levels of impunity and corruption we had reached, you have to recognize the advances.”

“Lozoya is the smoking gun. That’s what they’re promising us,” he said. “Let’s see what happens.”