Special counsel Robert Mueller is investigating a payment made to President Trump's foundation by a Ukranian billionaire for a speech Mr. Trump made in 2015 during his presidential campaign, according to the New York Times.

After federal investigators subpoenaed the Trump Organization, documents were handed over detailing a $150,000 payment made by Ukrainian billionaire Victor Pinchuk to the Donald J. Trump Foundation, the Times reported. Mr. Trump in September 2015 gave a 20-minute speech to a conference in Kiev, Ukraine, via video video link, commenting on everything from Syria, to having a "weak" president in Barack Obama, to his poll numbers. In the video, Mr. Trump frequently referred to Ukraine as "the Ukraine," generally an offensive reference to Ukrainians. The speech was a slightly awkward one, with Mr. Trump apparently experiencing a delay and audio problems, prompting some laughs from the audience.

Pinchuk is a Ukranian businessman and oligarch, who built his wealth particularly from steel.

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During the campaign, Mr. Trump and Republicans criticized former Secretary of States Hillary Clinton and former President Bill Clinton for accepting payments for speeches from foreign governments via the Clinton Family Foundation. Pinchuk also donated $13 million to the Clinton Family Foundation over several years, since 2006, according to the Times.

Mueller is investigating Russian election meddling and any ties to the Trump campaign. Mueller's probe doesn't appear to be slowing anytime soon. News that Mueller's investigation prompted the raid of Mr. Trump's personal attorney's office has frustrated Mr. Trump. The raid was carried out by the FBI's public corruption unit, CBS News' Andres Triay has reported.

Mr. Trump blasted the Cohen raid on Monday night, in thinly veiled comments to reporters, calling it a "disgraceful situation."

"So I just heard they broke into the office of one of my personal attorneys," the president said Monday night. "...It's a disgraceful situation. It's a total witch hunt. I've been saying it for a long time. I've wanted to keep it down. I've given over a million pages in documents to the special counsel. They continue to just go forward and here we are talking about Syria, we're talking about a lot of serious things...and I have this witch hunt constantly going on for over 12 months now. Actually it's much more than that. You could say right after I won the nomination it started."

Mr. Trump declined to rule out firing Mueller.

"I think it's a disgrace what's going on," the president also said Monday. "We'll see what happens. ... Many people have said you should fire him. Again, they found nothing and in finding nothing that's a big statement because you know the person who is in charge of the investigation -- you know all about that -- Deputy Rod Rosenstein he wrote the letter very critical of (fired FBI director James) Comey."