A non-partisan watchdog group has filed a Freedom of Information Act (FOIA) request in an attempt to determine whether conservative Supreme Court Justice Clarence Thomas improperly disclosed the use of private jets and a yacht.

Common Cause's FOIA request (PDF) to the U.S. Marshals Service encompasses travel on seven dates between 2007 and 2010.

"The request is aimed at determining whether Justice Thomas traveled on a plane owned by developer and political activist Harlan Crow on seven occasions over the past four years, and if so, whether those trips were properly disclosed," the group said in a statement.

A New York Times investigation last month raised ethics questions about the justice's relationship with Dallas real estate magnate Harlan Crow.

In 2004, Thomas was heavily scrutinized after he reported receiving $42,000 in gifts over six years. Since that time, he has not reported another gift.

But travel records obtained by the Times indicated that Thomas had probably used planes and a yacht owned by Crow.

"Americans are concerned, and rightfully so, over mounting evidence that our highest court is operating outside the ethical standards that apply to other federal judges," Common Cause President Bob Edgar said. "This is a serious matter. The Ethics in Government Act provides both civil and criminal penalties for willfully falsifying or failing to report required information on annual financial disclosures."

Common Cause has sent a letter to the American Bar Association urging them to join the FOIA request.