Filming has quietly begun under a shroud of secrecy on ABC's ambitious miniseries about the 9/11 terrorist attacks.

The project — which has never been formally announced by ABC and will air no sooner than January 2006 — stars Harvey Keitel (search) as John O'Neill, the FBI agent who hunted Al Qaeda for years and was killed in the attacks while working as head of security for the World Trade Center.

"We are trying to maintain as much accuracy, integrity and be as sensitive as we can in documenting an important series of historical events," producer Marc Platt told The Post.

The film focuses mostly on the events leading up to the attacks, which don't appear on screen until the very end, says Platt.

It is the first big Hollywood project to deal with the attacks and is based on the best-selling "9/11 Commission Report" (search), with former N.J. Governor and commission chairman Thomas Kean and other members of the panel serving as consultants.

"For us, having talked to Thomas Kean and the whole commission, we just felt it was a really important thing to bring to air," ABC chief Stephen McPherson told The Post yesterday at the TV critics press tour in L.A.

Meanwhile, Keitel is just one of 180 characters in the project, says Platt.

"He [O'Neill] was an unbelievably complex character. He really was a driven hero, and I don't think a lot of people know what he was about," McPherson says. "When you understand the journey he went through over all those years, you understand who this person was."

At the moment, ABC officials are calling the miniseries "Untitled Commission Report" and producers refer to it as the "Untitled History Project."

In a sad twist, the project began filming in Toronto on July 7, the day of the London terrorist attacks.

Platt says some scenes will be filmed in New York and Morocco. "That was simply because of production logistics and costs," says Platt. "There was nothing about us not wanting to be [filming] in New York City."