As days go by, the cast of characters in the Robert Menendez-Salomón Melgen saga keeps growing in the Dominican Republic, the location of one of the corruption schemes referenced in the April 2015 charges against the U.S. Senator from New Jersey and the Florida eye doctor.

At the heart of the Dominican side of the scandal appears to be a member of one of the island nation’s most influential political families, the Castillos.

The U.S. government alleges that Sen. Bob Menendez (D-N.J.), the former chair of the Senate Foreign Relations Committee, improperly tried to clear a Dominican government port security contract with a Melgen-owned company.

Melgen was born and raised in the Dominican Republic, and his uncle, Vinicio “Vincho” Castillo, founded the conservative FNP party and has been a political advisor to several presidents including the current President Danilo Medina.

Melgen’s first cousins are Vinicio and Pelegrin Castillo Semán, both FNP politicians. The latter is the Energy Secretary, while Vinicio junior is a congressman who has come out in Melgen’s defense more than once after the scandal first broke in 2012.

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In February 2013, in an open letter published in several newspapers in the Dominican Republic, Vinicio Castillo Semán said Menendez was a man of “irreproachable” behavior whom he had known only socially for 15 years.

“The only times that I’ve been at Dr. Melgen’s home with Sen. Menendez has been during parties or other social functions,” the long letter reads in part.

“It is my belief that this smear campaign,” he added, “is a result of conflicts with powerful sectors of the Dominican Republic, who flatly refuse to let the millions of containers that pass through our ports be scanned with high-technology X-rays.”

The Dominican portion of the Menendez-Melgen corruption case centers on a contract, signed in 2002 by the then-chief of the armed forces of the D.R., Gen. José Miguel Soto Jiménez, and a port security firm called ICSSI.

Under the terms of the contract – which covered a 10-year term that was automatically renewable – the company would X-ray containers entering or leaving the country at random for a per-container fee that ICSSI was to collect from the shippers.

The legitimacy of the contract has always been disputed, with company cofounders squabbling legally over ownership of the firm and critics of the contract – who suggest that its terms are overly generous to ICSSI and go against the country's economic interest – questioning whether Soto Jiménez had the authority to sign on behalf of the government.

Various decrees about the contract signed by former President Hipólito Mejía did little to clear up matters, and by the time Melgen took control of ICSSI in 2011, the contract issues were stalled, mired in the Dominican courts and in the court of public opinion.

Shortly afterward, Menendez became involved.

The U.S. indictment against Menendez and Melgen includes a series of e-mails between unnamed staffers in the senator’s office and in the Bureau of International Narcotics and Law Enforcement Affairs (INL) during the first half of 2012.

Menendez was much concerned with “the issue of a U.S. company attempting to sell a tracking and security system to the D.R. port authority, and suggesting they were being blocked by corrupt officials,” as described in a message from INL Assistant Secretary of State William Brownfield, whose is not named in the indictment but who has held that position since 2011.

Menendez demanded the stalled contract issue be resolved in less than six weeks. Brownfield said in another email that the senator threatened to call him to testify in an open hearing in Congress if no solution was reached. “I suspect that was a bluff,” he noted.

Six months later, in January 2013, Menendez staffers contacted people at U.S. Customs and Border Protection (CBP). The senator had been told that the agency was preparing to donate port-screening equipment to the D.R.

“Apparently there is some effort,” an individual identified in the indictment as Menendez’s chief counsel wrote, “by individuals who do not want to increase security in the D.R. to hold up that contract’s fulfillment. These elements (possibly criminal) want CBP to give the government equipment because they believe … [it] will be less effective than the outside contractor.”

The CBP’s response was that the agency had “not agreed” to donate any new equipment or expand its operations.

One U.S. law enforcement officer stationed in the D.R. with knowledge of the case who requested anonymity because he is not authorized to discuss the case told Fox News Latino, “Intelligence obtained from various sources indicates that [Vinicio] Castillo Semán was one of the sources providing information [to Menendez] about alleged corrupt Dominican officials blocking the port security contract between ICSSI and the Dominican government.”

Other sources confirmed that it was the congressman himself who fed information to Menendez and Melgen about the port security contract.

One highly-placed source in the D.R.’s justice system told FNL that in “several instances” Castillo Semán told Melgen and Menendez that corrupt Dominican officials were sabotaging the ICSSI contract by seeking a donation of screening equipment by the CBP.

“There is no doubt that Castillo Semán provided this information,” this source told Fox News Latino.

Fox News Latino repeatedly reached out to Congressman Castillo Semán’s office for comment, but as of the time of publication, calls and emails were not answered.

On Sunday, however, the lawmaker went on Dominican television and spoke on the subject. "My family has no connection of any sort in the allegations against Melgen," he said.

Among many in the D.R., however, there is a sense of disappointment about the congressman’s connection to the Menendez-Melgen case.

“It was more important to help his cousin on a business deal than to respond and show solidarity with the national interest,” a source in the Dominican government lamented to FNL.