Linda Spillers for The New York Times

Federal regulators charged the former chief executive of the nation’s largest pension fund on Monday with defrauding a private equity firm into paying $20 million to his close friend, the latest case to raise questions about suspected conflicts of interest at state retirement funds.

Federico R. Buenrostro, who was the top official at the California Public Employees’ Retirement System, or Calpers, from 2002 to 2008, was sued by the Securities and Exchange Commission, which accused him of steering contracts to clients represented by his friend, Alfred J. R. Villalobos.

The S.E.C. claims that the two men orchestrated a scheme to defraud Apollo Global Management, a New York-based private equity firm. A complaint, filed in Federal District Court in Nevada, claims that they fabricated documents to give Apollo the false impression that Calpers had approved the payments to Mr. Villalobos.

“Mr. Buenrostro strongly denies that he was involved in any type of fraudulent or illegal conduct,” said William H. Kimball, a lawyer for Mr. Buenrostro.

Mr. Villalobos, who filed for personal bankruptcy last year, could not be reached for comment. His lawyer could not be immediately identified.

The corruption charges against Mr. Buenrostro and Mr. Villalobos are connected to a nationwide scandal involving the role of middlemen who collected fees from money managers that won multibillion-dollar contracts from pension funds. Regulators from California, New York and other states have cracked down on widespread influence peddling into how pension funds like Calpers decide where to invest.

Among the pension funds at the center of the scandal was Calpers, which has about $235 billion in assets under management. Mr. Villalobos, a former Calpers board member, reaped at least $58 million in fees from money managers by helping to secure investments from Calpers.

Apollo paid at least $48 million to Mr. Villalobos to help it secure investments from Calpers, according to Calpers documents. Some of those payments came after July 2007, when Calpers paid about $600 million for a 9 percent stake in Apollo.

After reports of the controversial payments to Mr. Villalobos surfaced in 2009, the pension fund commissioned a Washington law firm to investigate allegations of conflicts of interest related to how it awarded business. The report absolved Apollo of any wrongdoing, instead pinning the blame on Mr. Villalobos.

Mr. Villalobos, the report said, turned Mr. Buenrostro into “a puppet” by lavishing him with gifts and promises of a lucrative job once he left Calpers. The two men both live in Zephyr Cove, Nev., a small town on Lake Tahoe just across the California border.

Mr. Buenrostro took a job at Mr. Villalobos’s company, Arvco Capital Research, less than two months after resigning from the Calpers board in 2008, according to the report.

The California attorney general filed a lawsuit against the two men in 2010. The case is continuing.

In Monday’s complaint, the S.E.C. detailed how Mr. Villalobos and Mr. Buenrostro had supposedly defrauded Apollo. Mr. Buenrostro was said to have signed blank sheets of fake Calpers letterhead, which Mr. Villalobos then used to generate phony letters demonstrating that Calpers had been aware of the payments to Mr. Villalobos. They did this for at least five different Apollo investments, regulators said.

“The allegations described in the complaint filed today by the S.E.C., if true, are troubling,” said Charles V. Zehren, a spokesman for Apollo, which is run by the investor Leon Black. “We have cooperated fully with the regulatory agencies investigating this matter, and will continue to do so.”