Three Democratic senators have launched an inquiry into allegations of financial improprieties at the National Rifle Association, the latest in a series of official investigations and regulatory complaints buffeting the organization.

The members of the powerful Senate Finance Committee — Robert Menendez of New Jersey, Sheldon Whitehouse of Rhode Island, and Ron Wyden of Oregon — sent letters on May 3 to Wayne LaPierre, the group’s leader and executive vice president; former NRA President Oliver North, and Revan McQueen, the CEO of the organization’s longtime marketing and PR firm, Ackerman McQueen.

The Finance Committee has oversight of issues related to taxation, including tax-exempt organizations like the NRA. The letters request relevant information concerning financial misconduct allegations, including “any internal or third-party audits, reviews, or investigations,” and set a May 16 deadline for responding.

The probe comes on the heels of an investigation by The Trace and The New Yorker documenting how the organization’s leadership and preferred vendors, including Ackerman McQueen, had extracted hundreds of millions of dollars from the nonprofit’s budget. Following the revelations, the attorney general of New York, where the NRA is chartered, opened an investigation into the NRA’s nonprofit status, asking the group and several affiliated groups to preserve financial records.

Amid the fallout, in-fighting has broken out among the NRA’s leadership. On the eve of the gun group’s convention in Indianapolis, LaPierre alleged that North had tried to pressure him to resign by threatening to release a document detailing new financial and harassment allegations allegedly prepared by Ackerman McQueen.

Most of the contents of North’s letter, including details of LaPierre’s $200,000 wardrobe and $240,000 in undocumented high-end travel expenses, have since been leaked to the press. But not before North was ousted as president. As the convention got underway, Richard Childress, the NRA’s first vice president, read aloud North’s resignation letter, in which North said that he had launched a committee to scrutinize financial improprieties at the NRA.

“If true, the NRA’s nonprofit status is threatened,” North’s letter said. “There is a clear crisis that needs to be dealt with immediately and responsibly so the NRA can continue to focus on protecting our Second Amendment.”

On Monday, after a meeting of the NRA’s board, LaPierre was re-elected as executive vice president.

Separately on Friday, Senator Wyden’s office also sent out letters to former presidents of the NRA — David Keene, Allan Cors, and Pete Brownell — and John Frazer, the group’s general counsel and secretary, regarding his ongoing “investigation into Russian engagement with the National Rifle Association and related political activities.” Wyden’s office says the men had previously failed to make themselves available for interviews or provide documents and gave them two weeks to cooperate with his inquiry.

Read the senators’ letters here: