A top Environmental Protection Agency (EPA) official under congressional investigation for using a private email address to conduct government business is stepping down.

EPA Region 8 administrator James Martin is resigning this week, according to a press release from Sen. David Vitter (R., La.). Martin faces a congressional probe for allegedly using a private email account to circumvent disclosure requirements.

Bob Perciasepe, the current acting EPA administrator, also used a private account to send emails to other EPA officials, Vitter said his office has discovered.

"Region 8 administrator Martin is likely resigning this week in part because of the open investigation about his use of a non-official email account to conduct official business," Vitter said. "Now we know that Lisa Jackson's acting replacement, Bob Perciasepe, appears to have been doing the same thing to dodge the agency's mandatory record-keeping policy."

As reported by the Washington Free Beacon Tuesday, the EPA released the second of four tranches of emails on Friday from former administrator Lisa Jackson’s secret "Richard Windsor" email account.

Perciasepe sent an email to Jackson’s "Richard Windsor" account from his personal account.

An EPA spokesperson said Martin was resigning for "personal reasons."

Vitter said the investigation is far from over.

"EPA owes us all some answers about their absolute disregard for transparency, especially from their acting administrator or any potential nominee to be administrator," Vitter said. "There's a lot of information in these emails that warrant further investigations, but it is clear that EPA continues to abuse exemptions under FOIA law with significant redactions of information to avoid transparency."

Competitive Enterprise Institute senior fellow Chris Horner, who first noted the existence of secret accounts by top administration officials in his book, The Liberal War on Transparency, said he was "not all that surprised" by the developments.

"As I chronicled in the book, this crowd has moved government business, often activities they prefer not receive scrutiny, over to private email, private computers, and even privately owned and managed computer servers," Horner said. "Somehow such dead-drop and electronic safe-house ‘tradecraft,’ even using industry lobbies as their ‘cutout’ or go-between to avoid paper trails with liberal activist groups, is what they meant by ‘most transparent administration, ever.’"

Vitter and House Oversight and Government Reform Chairman Darrell Issa (R., Calif.) sent a letter in January to Martin, who the legislators say used a personal me.com email address to set up a meeting with a lawyer for the Environmental Defense Fund.

"We are concerned that your use of the me.com email account may be an attempt to circumvent the Federal Records Act, the Freedom of Information Act, and Congressional oversight," the lawmakers wrote.

The EPA denied any wrongdoing took place and said the agency "and the regional administrator have gone beyond any legal requirements in our efforts to ensure full transparency."

"As detailed in public filings, the regional administrator does not use his personal email account to conduct official business," an EPA spokesperson wrote. "That Mr. Martin responded to one email sent to his personal email account to confirm a meeting that appears on his official government calendar does not alter that fact."

Martin and Jackson’s alleged actions are at odds with multiple executive branch and agency directives.

"EPA has a clear and consistent policy framework against the use of nongovernmental email systems for official EPA business," the agency wrote to the Government Accountability Office in 2008.

EPA political appointees were instructed by the Obama administration in 2009 to "not use any outside email account to conduct official agency business."

The letter also reveals that the EPA sent an agency-wide email reminding employees the "EPA prohibits the use of non-EPA email systems when conducting agency business."

Martin’s email was produced in litigation between the Competitive Enterprise Institute and EPA. It was first reported in the Daily Caller.