Environmental Protection Agency Administrator Lisa Jackson has decided to resign her post as top environmental regulator, a move that comes as Congress applies pressure on her for complicating oversight of the EPA by using at least one secondary email address under an alias.

“I want to thank President Obama for the honor he bestowed on me and the confidence he placed in me four years ago this month when he announced my nomination as Administrator of the Environmental Protection Agency,” Jackson said in a statement today. “I will leave the EPA confident the ship is sailing in the right direction, and ready in my own life for new challenges, time with my family and new opportunities to make a difference.”

Lawmakers and the EPA inspector general wonder if Jackson used a private email account created under an alias to develop those policies in a way that would be invisible to watchdogs and congressional oversight.

“Our objective is to determine whether EPA follows applicable laws and regulations when using private and alias email accounts to conduct official business,” assistant inspector general Melissa Heist wrote to the EPA on December 13, 2012, in announcing that an investigation was under way.

House Energy and Commerce Committee investigators asked a similar question. “We seek to understand whether conducting business with an alias has in any way affected the transparency of the agency’s activities or the quality or completeness of information provided to the Committee,” they wrote on December 13.

Jackson used a private email under the alias “Richard Windsor” to correspond with EPA colleagues, a decision her staff defended by saying that her official email account received too many messages for her to use it efficiently.

“While we understand the need for a secondary account for management and communications purposes, your choice to use a false identity remains baffling,” several lawmakers from the House Committee on Science, Space and Technology replied. “We remain concerned about whether EPA has adequately preserved these records and provided appropriate responses to requests for these records. We also question whether responses to records requests sufficiently connect the alias accounts to the real individual.”

The EPA says there is nothing to worry about. “We said three weeks ago that we welcome any investigation,” an agency spokesperson told Politico, promising to cooperate with the investigation.

The New York Times indicates that Jackson’s resignation may be a commentary on Obama, “as many in the environmental movement are questioning Mr. Obama’s commitment to dealing with climate change and other environmental problems.” Jackson considered resigning last year when Obama reportedly spiked a proposed regulation due to concern that the damage to the economy could weaken his reelection hopes.