"More than two years after this problem was first discovered by White House staff," Waxman said, "the White House still has not identified the cause of the problem, determined the volume of emails lost, or developed a plan for restoring those emails that were lost."

In a related controversy, House investigators have determined that hundreds of thousands of e-mails from former presidential adviser Karl Rove and other White House aides are missing because they were sent using external accounts set up by the Republican National Committee.