Story highlights Federal agents raid Florida clinic of Dr. Salomon Melgen for the second time

They don't say why ,but Melgen is being probed for Medicare and Medicaid billing practices

He's also been a donor to Sen. Robert Menendez, who questioned an audit into Melgen

Menendez flew on Melgen's plane, paid him back after the fact

Federal agents on Tuesday raided a South Florida office of Dr. Salomon Melgen, an eye doctor whose past issues with authorities had already entangled Sen. Robert Menendez

With yellow crime tape strung up outside, men and women lugged box after box of materials from Melgen's West Palm Beach office into awaiting minivans. Both members of the federal Health and Human Services department and the Federal Bureau of Investigation took part in the search, which FBI Special Agent Michael Leverock said was the second at the same Melgen clinic -- the other coming last January.

"As this is part of an ongoing investigation, (authorities have) no further comment/information at this time," said Leverock.

That means no official details as to what authorities were looking for. Even if it has nothing to do with his record as a generous political donor, Tuesday's raid suggests Melgen's legal troubles and perhaps their negative impact on Menendez, the senior U.S. senator from New Jersey are far from over.

The doctor's lawyer contended that Tuesday's search was the government's way of getting back at Melgen, after he sued over Medicare payments. Even so, Matthew Menchel, the attorney, said that Melgen will continue to work with authorities.

"While we believe that today's intrusion was in retaliation and there was no legitimate need for the search, the FBI's actions will not prevent Dr. Melgen from continuing his full cooperation with the government," Menchel said.

The doctor first came to the attention of many around Washington when The Daily Caller, a conservative website, published a report shortly before the November 2012 election citing several Dominican women who claimed they'd had sex with Menendez for money.

The New Jersey Democrat staunchly denied the accusation. And in March, Dominican police announced three women had been paid to claim -- falsely -- that they had sex with Menendez.

While that part of the story died down, the episode raised questions about why and how Menendez admittedly flew to the Dominican Republic three times in 2010 on Melgen's private plane.

One of those flights was on official business, while two others were for private reasons. The now two-term U.S. senator -- following his re-election last fall -- personally paid Melgen approximately $58,500 for the flights, admitting he had failed to pay in more timely, proper payments because "it unfortunately fell through the cracks."

Others have also brought up how Menendez raised concerns regarding a federal audit that showed Melgen had overbilled the federal government by $8.9 million in Medicare and Medicaid payments when treating patients.

Melgen, his wife Flor and his daughter Melissa have been generous donors to Menendez, his fellow Democrats and related causes in recent years. Yet in an interview last spring with Bloomberg, Melgen insisted that he "did not ask Sen. Menendez or any public official to intervene."

In the same interview, the doctor said he and Menendez had "become like brothers, like friends," talking to each other every week and seeing each other once a month until the first news broke.

The matter of Melgen's billing practices, meanwhile, has taken more twists and turns in recent months. The doctor sued the federal government in August related to his Medicare claims, with authorities asking to suspend his Medicare payments three weeks later, according to Melgen's lawyer. Just last Friday, Melgen filed a motion seeking an injunction to stop the government.