Figueroa also said that irregularities in vote tallies might eventually lead to the opening and re-counting of votes from as many as 50,000 polling stations, about a third of the 143,000 involved in Sunday's vote.
Lopez Obrador said Tuesday that his team had detected irregularities at 113,855 polling places, and called for a total recount.
"This is a scandal ... They bought millions of votes," Lopez Obrador said at a news conference, referring to the PRI. "Clearly, they far exceeded campaign spending limits ... this is a national embarrassment."
Lopez Obrador has refused to accept the preliminary vote tallies, saying the election campaign was marred by overspending and favorable treatment for Pena Nieto by Mexico's semi-monopolized television industry.
Many Mexicans also questioned why pre-election polls showed Pena Nieto with a double-digit lead, roughly twice the margin he really won by. With 99 percent of the vote tallied in the preliminary count, Lopez Obrador trailed by six percentage points.
The narrower-than-expected margin has fueled suspicion among Lopez Obrador's followers about the fairness of the vote, and he refused Monday night to concede defeat, just as he did when he lost a razor-thin race in the 2006 presidential election and set off months of political unrest. This time, he has not called his followers into the streets to protest.
Lopez Obrador argued from the start of the campaign that pollsters were manipulating surveys to promote the idea that the PRI candidate was far out in front.
Pollsters deny that, saying they believe some voters switched to Lopez Obrador in the final week, a period when publication of new polls is banned by law.
Lopez Obrador said he would not accept the preliminary election results reported by the Federal Electoral Institute and would wait until Wednesday, when the official results are to be announced, before deciding what he will do.
"We will not accept a fraudulent result," Lopez Obrador said.
Calls from conservative and pro-business groups mounted for Lopez Obrador to accept the results.
"We hope that the leftist candidate ... will also adhere to legality and recognize the official results once the authorities issue a final result," the Mexican Employers' Confederation said in a statement.
Associated Press writer Mark Stevenson contributed to this report.