WASHINGTON — Sen. Robert Menendez is being investigated by a Miami federal grand jury for his role in advocating for the business interests of a wealthy donor and friend, The Washington Post reported Thursday.

A story on the newspaper's website said that as part of the probe federal agents have questioned witnesses about the interactions between Menendez, D-N.J., and Florida ophthalmologist Salomon Melgen. The newspaper said the grand jury also issued subpoenas for Melgen's business and financial records. The newspaper cited unidentified people it said were familiar with the probe.

Federal agents have not contacted Menendez, one person told the newspaper.

The Post said Melgen declined to say Thursday whether he knew anything about the investigation, but he said any probe would find no wrongdoing.

Melgen attorney Kirk Ogrosky said the eye doctor is proud of his relationship with the senator and had no worries about any federal investigation.

Menendez and Melgen's overlapping interests have repeatedly raised questions in recent months. Menendez was compelled to reimburse $58,000 for two flights to the Dominican Republic aboard Melgen's private jet for personal trips in 2010 that he previously had failed to report, prompting scrutiny by the Senate Ethics Committee.

Menendez also has acknowledged contacting U.S. health agencies to question their billing practices and policies amid a dispute between Melgen and federal authorities. And Menendez was a key sponsor of a natural gas bill that could have aided a Melgen investment in a Florida company that markets a conversion system for natural gas truck engines.

Melgen has given more than $14,000 directly to Menendez's political campaigns since the late 1990s and, through his eye clinic, donated $700,000 last year to a "super" political committee that supported Democratic Senate candidates. The committee, in turn, spent $582,000 to back Menendez's re-election effort.

The investigation began with two disparate issues, according to people familiar with the matter who spoke with the Post. First, auditors had been reviewing allegations that Melgen was fraudulently overbilling Medicare for treating his patients. Melgen's attorney has said the doctor's billing was completely appropriate.

Then, in the fall, the FBI began looking into an anonymous tipster's allegations that Melgen had arranged prostitutes for Menendez in the Dominican Republic. Such an arrangement could constitute providing a favor or gift under the bribery statute that investigators have been reviewing, the Post said.

Lawyers for Menendez and Melgen have said the allegations involving prostitutes, which also were made by the conservative website the Daily Caller, were absurd. Menendez has said the allegations, which first surfaced during his re-election campaign last year, were part of a Republican smear campaign.

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  • The Numbers

    The House has 233 Republicans and 200 Democrats. Each party should pick up one more seat when two vacancies are filled. Going into the election, the GOP edge was 242-193. Senate Democrats will have a caucus of 55, including two independents, Angus King of Maine and Bernie Sanders of Vermont. Republicans have 45. That's a pickup of two seats for Democrats. <em>(Text <a href="http://www.huffingtonpost.com/huff-wires/20130103/us-new-congress-glance/">via the Associated Press</a>)</em>

  • Women

    The House will have 79 women, including 60 Democrats. At the end of the last session, there were 50 Democratic women and 24 Republican women. The new Senate will have 20 women members, an increase of three. That consists of 16 Democrats and four Republicans. The last Senate had 12 Democratic women and five Republicans. (Text <a href="http://www.huffingtonpost.com/huff-wires/20130103/us-new-congress-glance/">via the Associated Press</a>)

  • Freshmen

    With two vacancies to be filled, the House has 82 freshmen; 47 Democrats and 35 Republicans. As of the end of the last session, 87 of 103 freshmen were Republicans. The Senate will include 14 new faces, with nine Democrats and the independent King. Five are women. New senators include Brian Schatz, who was sworn in on Dec. 27 to fill the seat of the late Hawaii Democrat Daniel Inouye. (Text <a href="http://www.huffingtonpost.com/huff-wires/20130103/us-new-congress-glance/">via the Associated Press</a>) <em>(Pictured at left: Incoming House freshmen of the 113th Congress pose for a group photo on the East steps of the Capitol in Washington, Thursday, Nov. 15, 2012. AP Photo/Susan Walsh)</em>

  • African Americans

    The House will have 40 African-Americans, all Democrats. The number of Democrats is unchanged, although two Republicans will be gone: Allen West, R-Fla., lost his re-election bid, and Tim Scott, R-S.C., was appointed to fill the Senate seat of Jim DeMint, R-S.C., who is retiring. Scott will be the first black lawmaker in the Senate since Roland Burris, who retired in 2010 after filling the Illinois Senate seat of Barack Obama for almost two years. (Text <a href="http://www.huffingtonpost.com/huff-wires/20130103/us-new-congress-glance/">via the Associated Press</a>) <em>(Pictured at left: Rep. Tim Scott, R-S.C., who was appointed by South Carolina Gov. Nikki Haley to replace outgoing Sen. Jim DeMint, R-S.C., walks out of the Republican policy luncheon on Capitol Hill in Washington, Tuesday, Dec. 18, 2012. AP Photo/Susan Walsh)</em>

  • Hispanics

    The new House will have 33 Hispanics, with 25 Democrats and eight Republicans. That's up slightly from last year. The Senate will have three Hispanics: Democrat Robert Menendez of New Jersey, Republican Marco Rubio of Florida and Republican freshman Ted Cruz of Texas. (Text <a href="http://www.huffingtonpost.com/huff-wires/20130103/us-new-congress-glance/">via the Associated Press</a>) <em>(Pictured at left: Rep.-elect Joaquin Castro, D-Texas, speaks with members of the media after a news conference with newly elected Democratic House members on Capitol Hill in Washington, Tuesday, Nov. 13, 2012. AP Photo/Pablo Martinez Monsivais)</em>

  • Other Minorities

    The new House will have nine Asian Americans, all Democrats. There are two American Indians: Tom Cole, R-Okla., and Ben Lujan, D-N.M. (Text <a href="http://www.huffingtonpost.com/huff-wires/20130103/us-new-congress-glance/">via the Associated Press</a>) <em>(Pictured at left: Sen.-elect, current Rep. Mazie Hirono, D-Hawaii, and her husband, Leighton Oshima ride the Senate Subway on Capitol Hill in Washington, Tuesday, Nov. 13, 2012. AP Photo/Harry Hamburg)</em>

  • Other Facts

    According to CQ Roll Call newspaper, the average age of House members in the 113th Congress is 57; the average age of senators is 62. It estimates that the House will include some 277 Protestants and Catholics, 22 Jews, two Muslims and two Buddhists. The Senate will have 80 Protestants and Catholics and 10 Jews. The House will have its first Hindu, Rep.Tulsi Gabbard, D-Hawaii. Senate freshman Mazie Hirono, also of Hawaii, will be the Senate's only Buddhist and its first Asian American woman. Also for the first time, white men will be a minority among House Democrats. (Text <a href="http://www.huffingtonpost.com/huff-wires/20130103/us-new-congress-glance/">via the Associated Press</a>) <em>Pictured at left: Rep.-elect Tulsi Gabbard, D-Hawaii is seen on stage during a news conference with newly elected Democratic House members, on Capitol Hill in Washington, Tuesday, Nov. 13, 2012. AP Photo/Pablo Martinez Monsivais)</em>