A prominent board member resigned from the National Rifle Association this week, the sixth to step down in recent months, deepening the upheaval at the longtime gun rights powerhouse.

The departure of Richard Childress, a well-known NASCAR team owner based in North Carolina, came after he and then-NRA President Oliver North privately urged the group's leaders in a letter this year to more carefully review spending decisions under chief executive Wayne LaPierre, particularly legal fees totaling tens of millions of dollars.

In his resignation letter Monday, Childress made no mention of those issues and emphasized that he had chosen to leave the NRA board to focus on his private business.

He said he was resigning from the board and all NRA committees he served on effective immediately, "with great regret and a heavy heart." Childress wrote that he had "reached the point" where he could no longer fully commit his time to the organization.

"My hope is that the NRA will move forward with a focus on its important mission of defending the Constitutional rights of law-abiding citizens as provided in the Second Amendment, promoting firearm and hunting safety, enhancing our shooting sports and educating the general public about firearms," Childress wrote in his letter, which was first reported by NPR.

NRA spokesman Andrew Arulanandam did not respond Tuesday to requests for comment on the latest resignation.

Childress' exit comes amid a period of extraordinary turmoil at the gun rights organization, which is facing intense scrutiny from rank-and-file members and regulators about revelations of lavish spending by LaPierre and top vendors.

Democratic attorneys general of the District and New York are now investigating the NRA's tax-exempt status.

Last year, the NRA cut funding for its core missions of firearm safety and education while increasing its spending on legal fees, travel and entertainment. The Washington Post has also reported that NRA money flowed to 18 members of the group's 76-member board of directors, which is tasked with overseeing the nonprofit's finances.

NRA officials have defended their spending as appropriate.

"Our financial house is in order - we aren't going away," read a May 22 letter to members signed by the group's new president, Carolyn Meadows, and 11 other board members.

But the battle over the group's spending and leadership has fueled a spate of departures in recent months. In April, North was ousted as president after trying to force out LaPierre, saying as he stepped down that the group's finances were in "a clear crisis." Top lobbyist Christopher Cox resigned after he was accused of participating in an alleged extortion scheme with North to push out LaPierre.

Pete Brownell, a onetime NRA president who heads a major supplier of firearms accessories, left in late May.

At the beginning of this month, three NRA board members who had sought more information about the group's spending practices resigned. The three members - Esther Schneider of Texas, Sean Maloney of Ohio and Timothy Knight of Tennessee - said they had been stripped of their committee assignments.

"While our belief in the NRA's mission remains as strong today as ever, our confidence in the NRA's leadership has been shattered," they wrote in a letter to NRA officials.

A fifth board member resigned Aug. 12, when Julie Golob, a professional sport shooter and a strong public advocate for gun rights, announced she was ending her service before the end of her three-year term.

Golob, a regular on shooting shows who has won multiple competitions and is an advocate for women's use of firearms, did not state a reason for her departure in a note posted on her website.

Steve Pincus, an NRA member and firearms trainer who has been helping to lead an effort to reform the board, said he expects more resignations before the group's board meeting next month in Anchorage. "I'll be surprised if 50 show up," he said.

Pincus is advocating for mandatory attendance for board members at meetings, a smaller board and term limits.

Opposition to LaPierre has been building for months following revelations that he spent hundreds of thousands of dollars in NRA money on clothes and luxury travel. NRA officials have defended those expenses as necessary for media appearances and fundraising.

This month, The Post reported that the NRA chief had wanted the organization to buy him a multimillion-dollar mansion in a gated community in the Dallas area last year, a purchase that did not go through. NRA officials have said the real estate deal was an investment plan pushed by its now-estranged public relations agency, Ackerman McQueen.

While the NRA has also blamed gun-control advocates for fanning opposition to LaPierre, conservatives and NRA members have been increasingly vocal in their criticism of the group's leadership.

Fox News host Steve Hilton on Sunday called LaPierre "an odious little grifter," adding, "it's time for him to go."

“For years, Wayne has taken NRA members’ money to live the life of a king, but he’s not a king,” Hilton said. “He’s the head of a nonprofit trusted by millions to use its funds to secure constitutional rights.”