Federal officials investigating Mayor Vincent C. Gray’s 2010 campaign have recently focused on allegations that campaign staff members encouraged some supporters to donate by promising that their contributions would not be reported, several people familiar with the investigation said.

The staff members allegedly targeted city employees and contractors who feared that the administration of then-mayor Adrian M. Fenty (D) would retaliate if it knew they had contributed to his political rival’s campaign, said three people who spoke on condition of anonymity to speak candidly about the investigation.

FBI investigators are trying to determine whether donations were concealed in violation of campaign laws, according to several people familiar with the investigation. City law requires all campaign donations to be disclosed with the Office of Campaign Finance.

Separately, The Washington Post has discovered numerous discrepancies between an internal campaign list of nearly 2,750 contributions and reports the Gray campaign filed with OCF. The names of dozens of those donors could not be found in documents filed with the agency, according to a Post analysis.

“I can confirm that my clients, like several others, have been contacted by the FBI in connection with the Gray investigation and that both of them were open, honest and forthright in their interviews and are mere witnesses — not targets or subjects of the investigation. I remain confident that their status will not change,” Bolden said in a statement. “Furthermore, my understanding of those interviews is that neither of them knew very much and have little, if any, information to provide the U.S. Attorney concerning the Gray campaign’s fundraising operation.”

Attorney Thomas C. Green, who represents Lorraine Green, no relation, declined to comment.

If the solicitation took place, it is unclear which campaign staffers sought supporters, how many donors were contacted or how much money was raised. But those alleged contributions are separate from the funding of what federal prosecutors have called a “shadow campaign” — an illicit $653,800 get-out-the-vote operation on behalf of Gray.

“People on the [campaign] finance committee would say so-and-so can’t give,” said a committee member who requested anonymity to speak freely. “Other people were apprehensive about giving to Gray. . . . That would happen. Frankly, people were worried about retribution. That happens in any campaign.”

The committee member remembers discussions about lagging campaign fundraising but was not aware of an effort to solicit secret donations.

“They have been asking questions,” said another person familiar with the investigators’ new focus, adding that the queries have been about alleged unreported donations.

Lists of contributors

The internal campaign document The Post obtained lists 2,744 contributions to Gray’s campaign from late March 2010 through mid-August of that year but does not list donation amounts. The Post found about 60 contributors whose names and addresses could not be found in OCF documents.

They do not appear to be the contractors and city employees allegedly solicited by campaign staffers, which sources said are the subject of the federal probe. Instead, they gave much smaller amounts, between $25 and $50, according to interviews with a half dozen contributors.

If additional unreported donations were made to Gray’s official campaign, they would violate city campaign finance law, like the “shadow campaign.” It would also call into question the campaign’s portrayal of itself as the financial underdog in the contentious 2010 contest, based on campaign finance reports showing that Fenty out-raised Gray more than 2 to 1.