Republican presidential nominee Donald Trump often questions how his Democratic rival, Hillary Clinton, has operated the Clinton Foundation. But there may be even more unanswered questions about his own charitable foundation, including how he uses other people's money to make donations that raise eyebrows and could be illegal.

Trump started the Trump Foundation in 1987, and there are some strange things going on with it, The Washington Post reported.

Consider the findings of the Post's investigation, which is based on "examinations of 17 years of tax filings and interviews with more than 200 individuals or groups listed as donors or beneficiaries":

• Most of the money the foundations uses for charity comes from other wealthy donors, not Trump himself.
• The foundation has taken donations from other foundations and used it to make payments to still other groups, with Trump taking the credit.
• The Trump Foundation has also "been used for political purposes, which is against the law," the Post reported.
• Trump's foundation "appears to have repeatedly broken IRS rules, which require nonprofit groups to file accurate paperwork," the Post reported.

"Nearly all of its money comes from people other than Trump," the Post reported. "In tax records, the last gift from Trump was in 2008. Since then, all of the donations have been other people's money — an arrangement that experts say is almost unheard of for a family foundation."

Once Trump has received money from other donors, he typically donates that money to other charities or makes a number of other charitable investments.

As for the foundation's political activities?

A conservative activist group that filed a federal lawsuit against New York state Attorney General Eric Schneiderman received $100,000 from Trump's foundation in 2014. At that time, Schneiderman was suing Trump for fraud over Trump University, according to Yahoo!.

As we've already reported, Trump's foundation also donated $25,000 to a super PAC backing the campaign of Florida Attorney General Pam Bondi in 2013. Bondi had said her office was reviewing the fraud charges filed in New York against Trump University at that time. After the donation, Bondi dropped the inquiry into Trump U, The New York Times reported.

Beyond the political activity, it appears Trump has used money from his foundation to buy himself gifts. The Washington Post reported that Trump used $20,000 to buy a 6-foot portrait of himself and $12,000 for a football helmet autographed by NFL player Tim Tebow.

The Trump campaign declined to comment to the Post on its findings about the Trump Foundation.

ATTN: reached out to the Trump campaign for comment on this story and will update it when we hear from it.