When federal agents looking into health care fraud searched the office of Florida eye doctor Solomon Melgen in January 2013, they found references to U.S. Senator Robert Menendez scattered all around.

There was a black notebook, found on the bottom shelf of a bookcase, according to court records. In it, agents found what "looked to be a ledger of prostitution activities," FBI Special Agent Gregory Sheehy wrote in a court affidavit. Menendez's name also appeared in the notebook, along with his phone number, Sheehy wrote.

Sheehy later filed a more detailed affidavit that discussed the notebook, which he said included the names of numerous women, many "denoted simply by a single name in quotation marks." Most included a phone number next to their name, and a few mentioned a nationality, Sheehy wrote in the affidavit. There were also names of men, including Menendez, Sheehy said. The senator's name appeared on nine separate pages.

But in a motion Monday asking the judge to ignore the defense arguments, Justice Department lawyers say the hooker allegations "were not so easily disprovable as the defendants suggest."

Prosecutors used the defense claims to highlight previously undisclosed tidbits on Menendez's pursuit of younger woman.

The lawyers say their investigation resulted from "specific, corroborated allegations that defendants Menendez and Melgen had sex with underage prostitutes in the Dominican Republic."