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New York state's top prosecutor has made inquiries into Donald Trump's nonprofit foundation after questions about impropriety.

Attorney General Eric Schneiderman revealed the inquiry on Tuesday, telling CNN that "we have been concerned that the Trump Foundation may have engaged in some impropriety," although he did not go into detail.

"We’ve inquired into it and we’ve had correspondence with them," Schneiderman said. "I didn’t make a big deal out of it or hold a press conference. We have been looking into the Trump Foundation to make sure it's complying with the laws governing charities in New York."

A source familiar with the matter later told NBC News that correspondence with the foundation began on June 9.

In response to the news, Trump campaign spokesman Jason Miller released a statement calling Schneiderman "a partisan hack who has turned a blind eye to the Clinton Foundation for years and has endorsed Hillary Clinton for President."

He also called the inquiry a "left-wing hit job."

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The New York Attorney General's Charities Bureau, which oversees regulating nonprofits in the state, has asked about a $25,000 donation the Trump Foundation made in September 2013 to "And Justice for All" — a political group connected to Florida Attorney General Pam Bondi.

All nonprofits are barred from making politically-related contributions.

The Trump Foundation had 20 days to answer the Charities Bureau's letter. While the Trump campaign has previously declined to comment about the case, Trump during a campaign stop Monday in Ohio dismissed questions about the contribution, according to The Washington Post.

Trump paid a $2,500 penalty for failing to disclose the gift to the Internal Revenue Service, and representatives for the Trump Organization said he also paid back $25,000 to the foundation after the media began asking questions, the newspaper reported.

Trump Foundation treasurer Allen Weisselberg wrote in a June 28 letter to the attorney general that the error was first realized in March and was a "case of mistaken identity involving organizations with the same name."

Other questions by the attorney general had to do with the Trump Foundation supposedly paying $20,000 for a six-foot-tall portrait of Trump and four separate charities reportedly claiming they never received donations that the foundation said it gifted them.

The amounts that the foundation said it donated included a $10,000 contribution in 2008, $5,000 in 2010 and $10,000 in 2012, according to The Post, which reported about the discrepancies last week but issued an update on Tuesday noting that at least three of the charities in question did in fact receive money.

The portrait was referenced by President Barack Obama on Tuesday while he stumped for Hillary Clinton in Philadelphia.

"One candidate's family foundation has saved countless lives around the world," Obama said, referring to the Democratic presidential nominee's own charitable foundation run with former President Bill Clinton.

"The other candidate's foundation took money other people gave to his charity and then bought a 6-foot-tall painting of himself. I mean, he had the taste not to go for the 10-foot version," Obama quipped.

Schneiderman, a Democrat who has endorsed Hillary Clinton and raised money for her campaign, has previously gone after one of Trump's ventures.

In 2013, he filed a $40 million civil lawsuit against Trump University claiming the online school had fleeced would-be real estate investors.

That trial is going forward, and another fraud trial against Trump University in a San Diego court is scheduled to begin Nov. 28.