Federal investigators have ended their inquiry into a $135 donation casino giant Wynn Resorts gave to a university in the Chinese gambling enclave of Macau.

The Securities and Exchange Commission has concluded the investigation and is not pursuing enforcement action against the company, Karen Martinez, assistant director of the commission's Salt Lake Regional office, said Monday.

Speaking to The Associated Press from his boat on the Spanish island of Ibiza on Monday morning. Wynn said he never had any doubt federal investigators would clear the company.

"We were so sanguine that we never paid any attention to it; we had no exposure. It was a nonevent except for the damn newspapers," he said.

The six-month federal investigation has its origins in an ongoing battle between former friends and business partners Kazuo Okada and Wynn.

Okada used to be Wynn Resorts' single largest shareholder, but the company forcibly bought back his shares after it said it found that Okada made improper payments to overseas gambling regulators.

The two have traded accusations of unethical or illegal conduct during an extended legal, and seemingly personal, dispute.

In a 2012 letter to the Securities and Exchange Commission, Okada suggested that the company's $135 million donation to the University of Macau Development Foundation may have violated the Foreign Corrupt Practices Act— a law that bars U.S. companies from paying off officials to win business overseas.

He characterized the 2011 donation as "suspicious" and noted that the Development Foundation's lead trustee is also a member of the Macau government. He said that the donation coincided with Wynn's request for land to develop a third casino.

"I am at a complete loss as to the business justification for the donation, other than that it was an attempt to curry favor with those that have ultimate authority for issuing gaming licenses," Okada wrote.

Wynn Resorts has dismissed these charges. On Monday, Steve Wynn noted that Okada himself approved the gift. The Japanese billionaire was photographed representing the company in a ceremony to commemorate the donation.

"His ridiculous allegation about our gift, which all 16 of our directors approved, including him, the phony bum," Wynn said.

Okada is now under federal investigation himself for possible bribery in the Philippines.

A spokesman for Okada did not return calls and emails seeking comment.

In February, Nevada gaming officials concluded their own investigation into the donation and found no evidence of wrongdoing.

Wynn said he had never worried about the investigation damaging the company's reputation.

"The gaming industry in America has always been the subject of reports that were falsely researched and relied on rumor," he said.

"It never affects our business."

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Hannah Dreier can be reached at http://twitter.com/hannahdreier.