The FBI is investigating whether Egypt’s intelligence services might have been involved in the alleged bribery scheme described in the indictment of Sen. Bob Menendez and his wife, sources familiar with the matter told NBC News.

The counterintelligence investigation is in addition to the federal corruption case that accuses Menendez, D-N.J., of accepting hundreds of thousands of dollars in bribes, the sources said. As chairman of the Senate Foreign Relations Committee, Menendez helped oversee billions of dollars in U.S. aid to Egypt. He stepped down as chairman after he was indicted.

Investigators want to know whether Egyptian intelligence officials or their associates tried to gain access to Menendez through his wife, Nadine, the sources said.

According to a three-count indictment unsealed last week, Menendez accepted lavish bribes to wield his political position for the benefit the Egyptian government and to enrich a group of Egyptian American businessmen named as co-defendants.

The three New Jersey-based businessmen named as co-defendants, who are accused of providing bribes from “hundreds of thousands of dollars” in gold bars to a Mercedes-Benz convertible worth more than $60,000, are Wael Hana, Jose Uribe and Fred Daibes.

Uribe and Daibes are scheduled to be arraigned Wednesday, along with Menendez and his wife.

Hana was arraigned Tuesday and pleaded not guilty in federal court in New York City.

Hana’s lawyer, Larry Lustberg, denied that Hana has ties to Egyptian intelligence. He said that Hana and Nadine Menendez have been friends for years — and that the friendship will be part of their defense against the bribery charges.

Each of the defendants is charged with conspiracy to commit bribery and conspiracy to commit honest services fraud. Menendez and his wife face an additional charge of conspiracy to commit extortion under color of official right.

A lawyer for Nadine Menendez declined to comment when asked whether she might have been used by Egyptian intelligence officials. Menendez’s Senate office did not respond to requests for comment.

Frank Figliuzzi, an NBC News security analyst and former assistant director for counterintelligence at the FBI, said Menendez’s post atop the Foreign Relations Committee put him “in a bull’s-eye position for foreign intelligence services that are looking to have him make decisions in their favor, including military equipment and materiel decisions on funding.”

“The question here is whether this all started chronologically with the senator’s marriage, with the senator needing plausible deniability or an arm’s length from any transactions that might be happening with intelligence operatives,” Figliuzzi said. “All of that should be looked at from a counterintelligence perspective.”

Menendez has denied all of the charges and has resisted calls to resign.

The Egyptian Embassy in Washington did not immediately respond to a request for comment Tuesday night.