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.

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...