Supreme Court Justice Clarence Thomas has acknowledged that he erred in not disclosing his wife’s income on his financial statements in the wake of complaints raised by liberal advocacy group Common Cause.

In filings dated Jan. 21, Thomas sent seven similarly-worded letters to the Federal Committee on Financial Disclosure asking to amend his disclosure forms. In the letters, Thomas said he “inadvertently omitted” the information due to “a misunderstanding of the filing instructions.”

However Common Cause, a watchdog group that monitors government and industry, is still crying foul as officials question his explanation.

“Justice Thomas sits on the highest court of the land, is called upon daily to understand and interpret the most complicated legal issues of our day and makes decisions that affect millions. It is hard to see how he could have misunderstood the simple directions of a federal disclosure form,” said Common Cause President Bob Edgar in a statement. “We find his excuse is implausible.”

Common Cause pointed out to the Judicial Conference of the United States, the regulator for the judicial branch of government, that Virginia Thomas’ earnings while at the Heritage Foundation, a conservative advocacy organization, were not reported from 2003 to 2007. During that period, Common Cause indicates that Thomas earned $686,589, a salary of $120,000 or greater each year.

Common Cause indicates that she left Heritage in 2008; but in 2009 Liberty Central, an organization with strong ties to the Tea Party Movement, which she co-founded, paid Thomas for services, according to its chief operating officer Sarah Field.

Neglecting to disclose this information would violate the Ethics in Government Act of 1978, which requires all federal officials, including Supreme Court justices, to disclose their spouse’s income. Thomas indicated “none” under the latter category on his disclosure forms from 2003 to 2009.

Common Cause on Jan. 20 also requested the Justice Department investigate the “apparent involvement” of both Thomas and Justice Antonin Scalia in what it calls “strategy sessions” hosted by Koch Industries in January 2010. Koch Industries, one of the nation’s largest privately-held companies owned by the Koch brothers, conservative magnates, is the umbrella for such companies as Georgia-Pacific.

Koch sent out a description for its next program in Palm Springs, Calif., entitled “Understanding and Addressing Threats to American Free Enterprise and Prosperity” which states:

“This action-oriented program brings together top experts and leaders to discuss –and offer solutions to counter – the most critical threats to our free society. …Past meetings have featured such notable leaders as Supreme Court Justices Antonin Scalia and Clarence Thomas; Governors Bobby Jindal and Haley Barbour; commentators John Stossel, Charles Krauthammer, Glenn Beck, and Rush Limbaugh; Senators Jim DeMint and Tom Coburn; and Representatives Paul Ryan, Mike Pence, and Tom Price.”

Common Cause claimed that sitting justices meeting with the second largest private corporation in the U.S. raises questions about whether the two jurists should back away from upcoming cases involving campaign financing by corporations.

Common Cause is asking the Judicial Conference to probe these matters and, if warranted, refer the issue to the Justice Department for enforcement under the Ethics in Government Act.

If found guilty, Thomas would become only the second justice in modern times to encounter ethics problems. Liberal Associate Justice Abe Fortas resigned in 1969 under pressure over financial and ethics issues.