Three days after the election of New Jersey Sen. Robert Menendez (D-N.J.), FBI agents are preparing to serve another round of subpoenas in a case involving Menendez's financial relationship with a community organization that received federal funding with Menendez's help.

The new subpoenas, from the U.S. Attorney's Office in New Jersey, will be served on a circle of people "associated with the senator and his office," a senior government source told ABC News.  Sen. Menendez is not considered a formal "target" of the investigation at this point, according to investigators who describe him only as "a person of interest" in the federal investigation.

"We reached out to the U.S. Attorney's Office some time ago to ask them if they needed any information at all from the senator. They told us that not only did they did not need any information but that he has no status [in the investigation] at all," said Matt Miller, spokesman for Sen. Menendez. 

The case, according to investigators, centers on $329,000 in rent payments made to then-Congressman Menendez from the community group at the same time he helped the group obtain federal funding.

The rent payments, over a period of nine years, were for a three-story brick house owned by Menendez in Union City, N.J. The group continued to rent the building from the new owners even after Menendez sold it.

The investigation first came to light on Sept. 8 when New Jersey's "The Star-Ledger" reported subpoenas had been served on the North Hudson Community Action Corp, a New Jersey not-for-profit group.

The earlier subpoenas were the basis for a round of negative political ads questioning Menendez' ethics. Menendez denied he was a part of any federal investigation during the campaign.

"Bob Menendez did nothing but support a well-respected agency in the exact same manner that he has supported other non-profits in the state," the senator's campaign said in statement when the investigation first became public. "We're troubled by the timing of this subpoena in the middle of a political campaign, but the facts are that the NHCAC has received federal funds for over 35 years."

Menendez says he had received informal approval in 1994 from the House Ethics Committee before entering into the lease arrangement with the community group.

"It is standard practice for members to seek and rely upon oral guidance given to them by House Ethics Committee staff. I am not aware of any instance where a House member received oral guidance from the House Ethics Committee staff and was later subject to discipline by the House Ethics Committee," said Marc Elias, counsel for Sen. Menendez.

As Election Day approached, government investigators appeared to halt their public evidence gathering efforts. Law enforcement officials say they wanted to avoid any activity that could be construed as interfering with the election process.

Official corruption probes have been a hallmark of Chris Christie's tenure as U.S. Attorney in New Jersey, and his office has won convictions or accepted guilty pleas in 97 public corruption cases since 2002. Twenty of those have been in Hudson County.

Even now, federal officials say, the New Jersey federal prosecutor is moving cautiously, serving only those subpoenas needed to build the groundwork for a potential criminal case and not seeking to obtain records from the senator or the senator's office.

Such document and record subpoenas would require the approval of the Justice Department in Washington, D.C.  Subpoenas to members of Congress are also carefully vetted to ensure they do not violate the "speech or debate" protections in the U.S. Constitution. In the aftermath of a May 2006 raid on the offices of Congressman William Jefferson (D-La.), the FBI was accused by some legislators of violating these protections.

Sources say that when the Menendez camp made its pre-election assertion that the senator was not under investigation, its members were walking a very "fine" line. The senator, serving out a term as an appointee of Gov. Jon Corzine, was not the "target" of an investigation, nor was he or were his dealings a "subject" of an investigation -- two careful distinctions drawn by prosecutors.

"We are confident that when the U.S. Attorney's Office completes its review, they will come to the same conclusion as the House Ethics Committee that this transaction was completely appropriate,"  Miller said.