New York's attorney general said he has opened an inquiry into the Donald Trump Foundation amid troubling media reports "to make sure it is complying with the laws governing charities". "My interest in this issue is in my capacity as regulator of non-profits in New York state," Eric Schneiderman said on CNN.
"We have been concerned that the Trump Foundation may have engaged in some impropriety."
A source told CNN that the inquiry was launched because of "troubling transactions that we've seen".
Schneiderman's office has been corresponding with the foundation about concerns since June.
The revelation of the investigation is particularly ironic given that Trump has hammered his rival Hillary Clinton on the Clinton Foundation, arguing that donors got special consideration from the former secretary of state.
The Washington Post and the Associated Press have extensively reviewed the records of the Donald J Trump Foundation. The Post uncovered contributions that were never received by their reported recipients.
The newspaper's most explosive finding to date involved a $25,000 (£19,000) contribution by the foundation in 2013 to a group supporting Florida attorney general Pam Bondi. Under US tax laws, charitable organisations are prohibited from contributing to political campaigns, and Trump was fined $2,500 by the IRS earlier this month.
At the time of the contribution Bondi was considering joining a $40m New York state lawsuit alleging that Trump had defrauded more than 5,000 individuals through Trump University, which was never licensed as an educational institution. Trump University is also the target of two class action lawsuits.
After Bondi received the donation, she opted not to join the legal action. She and Trump have insisted her decision was not influenced by the contribution.
Democrats on the House Judiciary Committee are now calling on US attorney general Loretta Lynch to investigate the Trump Foundation's donation to Bondi.
In its investigation, the Washington Post also cited tax records showing that Trump had not donated to his own foundation since 2008, and had, among other allegations, spent $20,000 worth of money earmarked for charitable purposes to buy a six-foot-tall painting of himself. Such expenditures are not allowed by charity organisations under US tax laws.
In response to news of the attorney general's investigation, Trump campaign spokesman Jason Miller called Schneiderman a "partisan hack who has turned a blind eye to the Clinton Foundation for years and has endorsed Hillary Clinton for president."
He added: "This is nothing more than another left-wing hit job designed to distract from crooked Hillary Clinton's disastrous week."