COLUMBIA -- U.S. Rep. Joe Wilson of South Carolina is being investigated for potential misuse of travel stipends, a probe that his staff said Tuesday is over six $2 souvenir goblets he bought in Afghanistan for military families.

Pepper Pennington, Wilson's communications director, said U.S. House Speaker Nancy Pelosi had provided no clear guidelines to congressmen about the daily subsidies they receive for overseas travel until after The Wall Street Journal published a report in March on the matter.

Pennington said Wilson, a Republican who represents the state's 2nd Congressional District and is a member of the House Armed Services Committee, was given $13 a day for travel expenses on a trip to Afghanistan.

He bought the cups as a small token of appreciation for Afghanistan veterans and their families, Pennington said.

"Given this nominal $13 daily per diem, it is highly unlikely that Wilson had any per diem travel expenses left to even spend on the souvenir cups priced under $2 for veteran constituents," Pennington said in a statement.

"We are clearly expecting this issue to be resolved favorably and swiftly over these tokens of appreciation for veterans priced under $2."

The Wall Street Journal reported Tuesday that congressional investigators are questioning six lawmakers, including Wilson, as part of a probe referred to the U.S. House ethics committee. The findings won't be made public until after the November general election, according to the report.

Wilson faces Democrat Rob Miller, a Marine Corps combat veteran, in November. Miller lost to Wilson in 2008, but the race this year is considered to be much more competitive. The 2nd District stretches from the Lowcountry to the Midlands.

Miller condemned Wilson on Tuesday for spending the stipend on items other than daily expenses. He called for congressional reforms to rules governing daily per diem payments at a campaign stop in July.

He wants members to be banned from using taxpayer money on gifts and from pocketing any leftover cash. Also, Miller said that the government should require a searchable, online database detailing all per diem payments and how congressmen spent the money.

"I think this is another reason that shows Joe Wilson represents everything that's wrong in Washington," Miller said. "Cleaning up ethics in Washington is one of my highest priorities."

The Wall Street Journal first reported the issue in March. Wilson told the paper that he bought the shot glass-size marble goblets in Kabul.

An examination by the newspaper found that in the past two years, hundreds of lawmakers spent a total of 5,300 days visiting 130 foreign countries on taxpayer-funded trips. Congressmen received between $375,000 and $625,000 in per diem cash over that period.

There's no documentation for how those funds were spent. Congress also doesn't keep any record of the amount of per diem that is returned to the government.

Estimates for Senate travel couldn't be calculated.

Lawmakers could be issued sanctions or the matter may be resolved by requiring them to reimburse the government, according to the article. Investigators also could decide that the rules have been too murky.

Pelosi updated travel rules in May that included a requirement that lawmakers return excess travel funds.