A federal grand jury is investigating former New Mexico Gov. Bill Richardson over possible campaign-finance violations stemming from his 2008 presidential run, including allegations that he arranged for supporters to pay off a woman who planned to say they had engaged in an extramarital affair, according to people familiar with the inquiry.

Several of Richardson's close associates have been granted immunity from prosecution in exchange for their testimony before the panel in Albuquerque, the people said. The panel is one of several grand juries in recent years to examine aspects of Richardson's administration and campaigns.

Richardson, 64, who is at an environmental conference in Nigeria this week, did not return repeated calls and emails seeking comment. His office declined to provide the names of his lawyers.

Jennifer Poersch, the senior finance adviser for his presidential campaign, said that she was "not aware of any wrongdoing or any inappropriate fundraising," and that she hadn't been called to testify before the grand jury.

The US attorney's office in Albuquerque would not confirm or deny the grand-jury probe. Those familiar with the case said that indictments were possible in coming weeks, but that the inquiry could conclude with no charges filed. The probe was first reported by the Albuquerque Journal two weeks ago.

The investigation comes at a difficult time for New Mexico Democrats, who are struggling to regroup after losing the governor's mansion -- Richardson could not run again because of term limits -- and a number of state legislative seats in 2010. The state is considered a key battleground in the 2012 presidential election and also may be crucial to control of the US Senate, as both parties fight for a vacant seat now held by retiring Democratic Sen. Jeff Bingaman.

The most explosive matter under investigation involves allegations by a former member of Richardson's inner circle. That individual said Richardson's political allies gave $250,000 to placate a woman who was considering suing the governor in 2007, exposing their alleged extramarital affair, according to people familiar with the federal probe. The woman's identity has not been disclosed, and the type of suit considered has not been confirmed.

Those familiar with the case say prosecutors are seeking to prove that the alleged payment to the woman was a de facto campaign contribution to Richardson, intended to further his bid for higher office. In that case, the alleged payment could have violated federal campaign-finance law if it was not reported and did not comply with limits on political donations.

Federal investigators are piecing together a paper trail of checks and international wire transfers surrounding the alleged payment to the woman, some people familiar with the matter said. The grand jury is also examining other alleged financial irregularities, as well as whether Richardson used his clout as governor to push through a controversial road project that benefited a major political contributor, the people said.

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