Virginia’s attorney general has appointed an outside prosecutor to investigate Gov. Bob McDonnell’s financial disclosures, in a widening scandal over a political donor who wrote a $15,000 check for the wedding of the governor’s daughter, and who was also a benefactor of the attorney general.

Kenneth T. Cuccinelli II, the attorney general, who is also the Republican candidate for governor this year, said on Wednesday that he named the outside prosecutor last November to look into Mr. McDonnell’s disclosures.

Mr. Cuccinelli said “information came to my attention” triggering the appointment of the prosecutor. His referral of the case to the Richmond commonwealth’s attorney, Mike Herring, whose role is similar to that of a district attorney, “was not a conclusion that any violation occurred,’’ Mr. Cuccinelli said in a statement.

The investigation came to light through a Freedom of Information Act request by The Richmond Times-Dispatch, which first reported it.

Mr. McDonnell and Mr. Cuccinelli, who are yoked in an awkward political alliance – the former a popular governor of a purple state and his would-be successor, a Tea Party favorite — have been swept up in controversy over their friendship with a Virginia businessman, Jonnie R. Williams Sr., who gave generously to both officials.

Mr. McDonnell did not report the $15,000 check for wedding catering in 2011, telling reporters when asked about it this year that it was gift to his daughter and exempt under Virginia law. The gift was disclosed by a former chef at the Executive Mansion, who accused Mr. McDonnell and Virginia’s first lady of “wrongdoing” and “abuse” in interviews with investigators from the state police, the attorney general’s office and the Federal Bureau of Investigation, according to court records.

The chef, Todd Schneider, was accused of stealing food from the Executive Mansion. In an apparent effort to avoid charges, he presented himself to authorities as a whistle-blower last spring. The F.B.I. has reportedly looked into whether a quid pro quo existed between Mr. Williams and the governor and his wife, Maureen McDonnell, who helped promote a dietary supplement made by Mr. Williams’s company, Star Scientific. Mr. Williams donated over $100,000 to Mr. McDonnells’s campaigns. The first lady promoted the supplement to investors and was host of a luncheon to introduce it at the Executive Mansion.

The governor has said he and his wife are friends of Mr. Williams, but he did him no favors.

Mr. Williams also gave gifts to Mr. Cuccinelli, including vacations at a lake house and a $1,500 catered Thanksgiving dinner, which Mr. Cuccinelli omitted from financial disclosure forms as required by law. He admitted the gifts, dating from 2010 and 2012, at a meeting with reporters on April 26, where he explained that he had inadvertently left them off his disclosure forms. The total value of gifts he accepted from Mr. Williams, including earlier ones he did disclose, is more than $18,000.

As Mr. Cuccinelli’s campaign for governor heats up against the Democratic nominee, Terry McAuliffe, he has sought to distance and distinguish his relationship with Mr. Williams from that of Mr. McDonnell’s. Mr. Cuccinelli said in April he had not seen Mr. Williams in nine months. At the time, he announced he was asking an outside attorney — Mr. Herring — to review his own statements of economic interest. “He did this on his own out of his desire for full transparency, so that another official would be in a position to provide an independent review,’’ Brian Gottstein, a spokesman for Mr. Cuccinelli in the attorney general’s office, said on Wednesday.