Golan Cipel's (search) alleged extortion demand for millions of dollars from New Jersey Gov. Jim McGreevey (search) was backed up by a threat to release embarrassing tapes and photos of him, The Post has learned.

The FBI wants to question McGreevey about the sensational gay sex case as early as next week and federal subpoenas for evidence are expected to be issued soon, sources said.

No specific date for the FBI questioning has been set but a McGreevey spokesman said, "We will cooperate fully."

But shortly after the speech, sources close to the governor claimed that Cipel had demanded $5 million in blackmail from him — or he would go public with their affair and file a sexual-harassment lawsuit.

Sources said yesterday that the key to the alleged extortion was compromising tapes and photographs of McGreevey that Cipel's side warned would be made public.

The contents of the material were not disclosed.

The FBI will investigate the possibility of extortion, as well as the conduct of McGreevey and his aides, sources said.

Cipel's lawyer, Allen Lowy (search), yesterday denied he or Cipel had threatened McGreevey with damaging photos or tapes.

"Never. It's absurd," Lowy said. "We haven't threatened anyone. We contemplated filing an action. After that, there were settlement discussions.

"Neither I nor Golan — who are the only people who know of the existence of any evidence — have described any kind of evidence except to say that we have corroborating evidence."

McGreevey's lawyer could not be reached for comment.

The FBI became involved in the case even before McGreevey's "I am a gay American" speech — when his office asked the feds to investigate Cipel for attempted extortion.

Cipel, now in Israel, said this week he would return to the United States in "a few weeks, so that justice will be done."

The Israeli has said he is not gay.

He met McGreevey when the former mayor of Woodbridge, N.J., toured Israel in 2000. After being elected governor, McGreevey hired Cipel as a $110,000-a-year homeland-security chief, but was forced to fire him amid public outrage.

Besides being a potential source of evidence against Cipel, McGreevey may be a target of investigators himself.

Federal officials will look into whether McGreevey attempted to obtain money to pay off Cipel, Fox News reported last week.

McGreevey, who is married and has two children, has remained silent about the scandal since his bombshell announcement last week that he was gay and had a homosexual affair — when he didn't mention extortion or the name of his lover.