But the aide asked if the United States government was planning to donate additional port security equipment to the Dominican Republic. The aide explained that if such a donation occurred, the Dominican government, perhaps under pressure from criminal elements there, might intentionally limit the use of the equipment so that drugs or other contraband could still flow through the country’s ports on the way to the United States.

Only by hiring the unnamed private contractor, the e-mail said, could the United States be assured that port security in the Dominican Republic would be enhanced.

“Apparently there are some efforts by individuals who do not want the increased security” in the Dominican Republic, the e-mail from Mr. Menendez’s office said.

“These elements, possibly criminal, want C.B.P. to give the government equipment,” the e-mail said, “because they believe the government use of the equipment will be less effective than the outside contractor. My boss is concerned that the C.B.P. equipment will be used for this ulterior motive.”

A Customs and Border Protection official assured Mr. Menendez’s staff that there was no immediate plan to deliver such equipment, so there was no need for the senator to intervene.

The agency “has not agreed to any extended operation in the Dominican Republic and has not provided any additional equipment,” the official wrote in reply, according to a federal official who read the e-mail exchange to The New York Times.

Late last month, investigators from the Department of Health and Human Services, with which Dr. Melgen has been involved in a dispute over Medicare billing, and the Federal Bureau of Investigation raided several of Dr. Melgen’s offices in Florida, and the senator has recently acknowledged intervening in the doctor’s billing dispute with the agency twice in the last four years. Dr. Melgen has denied any wrongdoing.