MIAMI - Federal agents descended on the clinic of a South Florida eye doctor close to Sen. Robert Menendez in January - a dark-of-night raid that soon spiraled into a string of national news stories about trips on the physician's private plane to the Dominican Republic and scandalous allegations of trysts with underage prostitutes.

Nearly a year later, the politically explosive allegations against the New Jersey Democrat have proved to be duds so far.

After subpoenaing records and witnesses, a federal grand jury in Miami has filed no charges against Menendez, a prominent Cuban American lawmaker, or his friend and major campaign donor, Salomon Melgen, according to several sources familiar with the probe.

But the grand jury is still reviewing evidence of Menendez's intervening with U.S. government officials on behalf of Melgen regarding the physician's billing dispute with Medicare and his port-security contract in the Dominican Republic.

And Melgen's legal problems are also far from over. Only two months ago, agents with the FBI and the Department of Health and Human Services conducted another search of Melgen's West Palm Beach clinic, Vitreo-Retinal Consultants - a second raid stemming from a separate ongoing battle with authorities over his prolific Medicare billing.

For Menendez, a pronouncement that the grand jury probe of the alleged trysts had come to naught would be a public vindication from what he once called a "fallacious" smear campaign orchestrated by a right-wing blog hoping to derail his reelection bid.

But that won't be coming.

Anonymous sources originally leaked that a grand jury investigation, which is supposed to be secret under federal law, had been launched to examine Menendez's conduct. But unless an indictment is issued, the results of federal grand jury investigations remain confidential - even when they don't bring charges against the targets of their probes.

The Senate Select Committee on Ethics, which could open an inquiry into Menendez's unreported gifts from Melgen, won't comment as a matter of policy.

Attorney Joseph DeMaria, a former federal prosecutor with the Miami law firm Tew Cardenas, said the principle of fairness has gotten lost in the process.

"There is something desperately wrong with our system when 'anonymous sources' leak secret grand jury information and then . . . the Department of Justice does not publicly clear the good name of the innocent person," DeMaria said Wednesday.

Menendez's office declined to comment, but he has said in the past that he welcomed the investigation.

Melgen's attorney, Kirk Ogrosky, with the Arnold & Porter law firm in Washington, also would not comment.

The Miami grand jury, which heard testimony from people associated with the politician and physician, focused on allegations that Melgen had arranged encounters with prostitutes in his native Dominican Republic while he and Menendez stayed at the doctor's seaside estate in the resort area of Casa de Campo.

But the grand jury found no basis to file any charge on that matter, according to sources.

Meanwhile, Melgen continues to face a separate Medicare fraud probe of his South Florida eye clinics. A federal grand jury based in West Palm Beach is investigating whether the ophthalmologist overbilled the government health-insurance program millions of dollars for medically unnecessary treatments.

In October, federal agents again raided his West Palm Beach clinic because of alleged inconsistencies in the medical and patient records he had turned over to authorities, according to sources familiar with the case. That investigation is based, in part, on a still-festering 2009 dispute between Melgen and Medicare over his billing and collecting more than $8 million for eye injections of a costly drug called Lucentis. The drug, manufactured by Genentech, is used to treat macular degeneration in mostly older adults.

In August, Melgen's legal team, Ogrosky and Matthew Menchel, both former federal prosecutors, sued the Department of Health and Human Services in Miami federal court to recover those Medicare payments for 2007 and 2008 - funds that the doctor had returned to the government during his administrative appeal.

Melgen's dispute with Medicare became one of the areas of inquiry for the Justice Department's Public Integrity Section in the Miami federal grand jury. Prosecutors, along with the FBI, investigated whether Menendez abused his official position to help Melgen while he made sizable campaign donations to the senator and provided him with free trips to the Dominican Republic, among other allegedly unreported gifts.

In 2009 and again in 2012, Menendez complained to top Medicare officials that it was unfair to penalize the doctor because the billing rules for administering Lucentis were ambiguous, the senator's aides told the Washington Post after it broke the story in February.

When Melgen, who has invested in a variety of businesses, needed help with a port security contract in the Dominican Republic last year, he turned to Menendez. The senator tried to get the State Department to revive the long-stalled, multimillion-dollar agreement at the Santo Domingo port with a company of which Melgen is part-owner.