Sen. Robert Menendez took at least one more trip to the Dominican Republic on the plane of a doctor and campaign donor than he has publicly admitted, according to sources who spent time with the New Jersey Democrat on the unreported visit.

Menendez, already under fire for his association with a South Florida doctor and political contributor whose offices were raided by the FBI on Jan. 29, visited the Dominican Republic on the doctor's private jet in 2008 and took part in an exclusive private golf charity tournament, the sources said.

Sources said Menendez flew on Dr. Salomon Melgen's plane to the Dominican Republic in April that year for the invite-only Sugar Open golf tournament at Casa de Campo, an exclusive vacation resort popular with American visitors. While there, Menendez stayed at the doctor's Casa de Campo villa, the sources said.

"He was on the doc's jet in 2008 -- that I guarantee," a source who attended the same event told FoxNews.com.

The senator did not report the 2008 trip in his annual disclosure forms, according to records reviewed by FoxNews.com. Depending on how the trip was funded, it's possible he was not required to. But claims Menendez attended the 2008 golf tournament would appear to conflict with a recent statement from the senator's office that he only flew on the donor's plane in 2010.

Representatives for both Melgen and Menendez did not return requests from FoxNews.com for comment. FoxNews.com communicated directly with the spokeswoman for Menendez on Monday, before this story was published, but the office did not comment on the record despite repeated requests.

After this story was first published, a statement denying that Menendez took an April 2008 flight with Melgen was sent to another news organization.

"This story is simply not true," the statement said.

The senator’s office has not responded to a question from FoxNews.com on whether any trip was taken on the doctor’s plane in 2008.

The annual Sugar Open event, which is hosted by sugar barons Alfy and Pepe Fanjul, "actually incorporates four evenings of entertainment as well as three days of golf," according the tournament website.

Menendez reportedly is under scrutiny by the Senate Ethics Committee, after failing to report previous trips on Melgen's private plane. The FBI is also thought to be investigating allegations from a purported whistleblower claiming Menendez traveled on the plane to procure prostitutes in the Dominican Republic, a claim the senator has denied.

Menendez has said he traveled on Melgen's plane three times, and did not initially report two of those trips -- personal trips to the Dominican Republic he took in 2010. Late last month, the senator said he reimbursed the doctor $58,500 on Jan. 4, after news reports prompted his staffers' review of the senator's travel logs.

"Senator Menendez has traveled on Dr. Melgen's plane on three occasions, all of which have been paid for and reported appropriately," a Menendez spokesman said at the time.

But sources who attended the 2008 tournament say that's not true.

"That's ridiculous, I heard him say that he only flew with him three times -- I saw him -- I absolutely guarantee the senator flew to the tournament in 2008 on the doctor's plane," one source told FoxNews.com.

A review of Menendez' financial disclosure forms shows that he did not report receiving any "reportable travel" worth more than $335 in 2008. It's possible he wasn't compelled to -- if a lawmaker makes a prompt reimbursement for travel expenses, he or she does not need to report it. However, lawmakers are otherwise expected to report travel if the value exceeds $335.

Melgen, in 2008, was using a different plane from the one Menendez flew on in 2010.

During the 2008 trip, Menendez and the doctor played golf in the tournament during the day. Sources described the senator's golf skills as "not good."

After they hit the links, Alfy Fanjul hosted his annual opening party. The tournament raises money for the Fanjul's Mission International Rescue charities that cater largely to underprivileged youth in the Dominican Republic. Its website has been "unavailable" since late last week.

"Everyone was there -- this is the who's who of billionaires, you can't even imagine how powerful these people are. No expense was spared," the source said.

The source said there was vintage Dom Perignon pouring from an open bar from afternoon until 1 a.m. There were about 500 people at the white tablecloth affair.

But Menendez and the doctor reportedly didn't attend. "They had their own party back at the doctor's house and I guess that party was better," the source said.

The Senate Ethics Committee declined to comment Friday on whether it was looking into or aware of any 2008 trips by Menendez to the Dominican Republic. Menendez earlier told reporters it was unfortunate that the two previously acknowledged trips in 2010 "fell through the cracks."

"I was in a big travel schedule in 2010 as the chairman of the DSCC" -- the Democratic Senatorial Campaign Committee -- "plus my own campaign getting ready for the election cycle," Menendez said.

Menendez' office said earlier that the senator's reimbursement was for the full cost of two flights on Melgen's plane to the Dominican Republic for personal trips in 2010.

The third flight he took on his friend's jet was to a campaign event in May 2010, and this was reported to the Federal Election Commission as $5,400 paid by the DSCC.

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FoxNews.com's Judson Berger contributed to this report.