(CNN) -- Barack Obama and his wife released their tax returns from 2000 to 2006 on his campaign Web site Tuesday, and the Illinois senator challenged Sen. Hillary Clinton to release hers.

Later, Clinton said she expected to release her tax returns "shortly, I hope within the next week."

Sen. Barack Obama is challenging Sen. Hillary Clinton to release her 2006 tax returns.

The Obama campaign said Clinton should immediately release her 2006 return, pointing to her decision to lend her campaign $5 million earlier this year.

The Obama camp also cited former President Bill Clinton's $20 million payout from supermarket holding company Yucaipa as a reason why the couple should release the documents.

The Obama campaign said that company invests in tax shelters in the Cayman Islands.

Clinton has promised to release 2007 returns sometime after this year's April 15 income tax deadline, just a few days before Pennsylvania's April 22 primary.

"Senator Clinton recently claimed that she's 'the most transparent figure in public life,' yet she's dragging her feet in releasing something as basic as her annual tax returns," Obama's communications director, Robert Gibbs, said in a statement. "Senator Clinton can't claim to be vetted until she allows the public the opportunity to see her finances -- particularly with respect to any investment in tax shelters."

But Clinton made her own challenge to Obama during a press conference in Greensburg, Pennsylvania.

"I am pleased that Senator Obama has released his tax returns. I think that's a good first step," she said. "Now he should release his records from being in the state Senate and any other information that the public and the press need to know from his prior experience."

According to the 2006 return, Obama and his wife, Michelle, earned over $991,000 in 2006 and paid $277,431 on the income.

More than half of the couple's income in 2006 -- nearly $507,000 -- came from royalties from his two books, "The Audacity of Hope" and "Dreams From My Father." In 2005, Obama earned over $1,210,000 from royalties, according to the couple's returns that year.

The Obamas earned nearly $431,000 in wages in 2006, according to the returns. The documents do not show what Obama or his wife individually earned, but the senator's salary was $165,000 that year.

They also earned more than $51,000 from Treehouse Foods, an Illinois-based food manufacturer. Michelle Obama served on Treehouse's board from June 2005 to May 2007.

Before 2005, the couple had more modest incomes. Between 2000 and 2004, they earned between $207,647 and $275,123 a year.

The returns were posted just minutes after the Clinton campaign attacked Obama for not releasing them.

"In the public record, there are 20 years of Hillary's tax returns, hundreds of thousands of pages of records from her time in the White House and countless other documents detailing her time in public life.

"Sen. Obama's record is far more opaque. Sen. Obama has not released his tax returns, except for 2006," Clinton spokesman Phil Singer said in a statement sent to reporters just as the Obama camp posted the Illinois senator's returns online. E-mail to a friend

CNN's Rebecca Sinderbrand and Alan Silverlieb contributed to this report.

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