Grand jury investigating Sen. Robert Menendez (D-N.J.), people familiar with probe say Melgen has provided Menendez with plane flights and hospitality at his Dominican vacation home, say people acquainted with their relationship. Last month, people with knowledge of the case said FBI agents were conducting interviews in the Dominican Republic and the United States concerning allegations against Menendez, including the role he played in advocating for the enforcement of the port security contract. A grand jury probe, which involves a prosecutor pursuing allegations with an eye toward possible indictment, typically represents a legal escalation, though it does not always lead to a prosecution. As part of the grand jury investigation, the three people said, federal agents have questioned witnesses about the interactions between Menendez and Melgen, who contributed $700,000 last year to Menendez and other Senate Democrats. The grand jury has also issued subpoenas for Melgen's business and financial records, according to two people briefed on the probe who, like the others, spoke on the condition of anonymity to describe an ongoing investigation. Federal agents have not contacted Menendez, according to a person familiar with the case. The senator, who chairs the powerful Senate Foreign Relations Committee, acknowledged in January that he had previously failed to disclose two free trips he took on Melgen's private plane to vacation with him in the Dominican Republic in 2010. Menendez said he wrote Melgen a personal check this winter for $58,500 to reimburse him for the plane rides. The Senate Ethics Committee is investigating why Menendez did not disclose the flights sooner. Menendez declined Thursday to say whether he knew anything about the investigation, but he said any probe would find no wrongdoing. "I welcome any review, because I believe, at the end of the day, that my actions have been appropriate," the senator said. And just as everything that gave rise to this was a smear campaign based on slanders that drove the original story, I believe that when any review reviews the facts, they will determine that I have acted appropriately at all times. Menendez said Thursday that his failure to disclose the flights was a "mistake" resulting from confusion about his frequent travel during 2010, when he was heading the Democrats" Senate campaign committee. "In the midst of all of that travel, flights that should have been filed in a different way weren't," he said.