JEFERSON CITY, Mo. — Missouri Democratic gubernatorial candidate Chris Koster released four years of tax returns Thursday, showing that his income from private investments has declined substantially during his current term as attorney general.

Koster released his tax information a day before the first scheduled candidates’ forum of the gubernatorial race and called upon his Republican rival, former Navy SEAL officer Eric Greitens, to do the same.

The Associated Press requested tax returns from Koster and Greitens shortly after they won their party primaries in August. Greitens’ campaign has said he will release some tax information but has not yet done so.

Koster’s federal tax returns show an adjusted gross income of nearly $241,000 in 2012, more than half of which came from his equity stake in Eagle Fund II, a St. Louis-based investment fund managed by his brother, Matthew Koster.

His adjusted gross income declined to nearly $172,000 in 2013 as he began a second four-year-term in office, about $136,000 in 2014 and nearly $114,000 in 2015 as his investment income tapered off. His 2015 income consisted mainly of his attorney general’s salary.

There is no requirement for political candidates to release tax returns, which are otherwise a private document for most individuals. But it has become a tradition to do so for presidential candidates, as well as for those running for some other top offices. Democratic presidential candidate Hillary Clinton has released her tax returns but Republican presidential nominee Donald Trump has not yet done so — an issue that prompted Clinton to suggest Trump was hiding something during a nationally televised debate earlier this week.

Koster campaign manager David Turner said Koster was releasing his tax returns “in the interest of full transparency.”

“We believe it is important to voters that Mr. Greitens release the same information the Attorney General has to give voters an understanding of any conflicts he may have if elected governor. The question is, will Eric Greitens blink now?” Turner said in an emailed statement.

Among other things, Koster’s tax returns show that he has claimed hardly any deductions for charitable giving.

Turner said Koster has given to charitable causes but simply chooses not to seek a tax break for most of his donations. He released a letter from the Southside Early Childhood Center in St. Louis confirming a gift of $4,700 in 2014.

Greitens founded The Mission Continues, a St. Louis-based charity that helps military veterans transition to the private sector by financing fellowships with local nonprofit groups. Previously released tax returns for The Mission Continues show Grietens initially worked without pay but eventually received a salary or $175,000 annually before stepping down as CEO in 2014.

Koster’s tax returns show he has been paying personal property taxes for a vehicle in Cole County, the home of the state capital. Koster also had owned a home Cass County, where he previously was a prosecuting attorney. But he sold that during his second term as attorney general and now rents apartments in Jefferson City and the St. Louis area, Turner said.