William Colby (AS Director of Central Intelligence) at committee hearings testifying before Congress (1975). (First Run Features)

Just weeks after the Sept. 11, 2001, attacks, Carl Colby says, he saw dramatic pictures from Afghanistan that made him think better of his father, arguably the most controversial director in the CIA’s stormy history.

They showed bearded CIA paramilitary operatives going into battle on horseback.

“When I saw CIA operatives disguised as Afghan tribesman going into battle against al-Qaeda, I thought, this is all going back to what my father was doing decades ago,” in Vietnam, Colby recalled over lunch recently in Georgetown. “I felt it was important to look more closely at his legacy.”

Of course, CIA operatives didn’t ride horses into battle in Vietnam, where William E. Colby made his mark, so to speak, first as station chief in the early 1960s, then as head of the infamous Phoenix program, which “neutralized” more than 20,000 suspected communists (or just people on somebody’s grudge list). But the idea — using guerrillas’ tactics to defeat them — preoccupied the CIA from the beginning of the Cold War through Colby’s tumultuous term as CIA director in the mid-1970s.

Still, it was Colby’s name that became synonymous with the Vietnam “assassinations.” And his young son felt the scorn.

“People would turn to me and say, ‘Your father was a murderer,’ ” Carl Colby says in his justifiably heralded new film about his father, “The Man Nobody Knew: In Search of My Father, CIA Spymaster William Colby,” opening at the Landmark E Street Cinema on Friday. “My immediate reaction used to be, ‘You don’t know what you’re talking about.’ And then I’d find myself thinking ‘Was he? Who was he, really?’ ”

Searching for the “real” father behind the mask, of course, is as old as Greece. That Carl Colby’s father had been a professional liar from the moment he parachuted behind Nazi lines in World War II until he died in a mysterious canoeing accident in 1996 at age 76, made the task all the more difficult.

“It was like peeling away layers of an onion,” says Carl, 60, at first glance a reincarnation of his father, whose enigmatic smile and seemingly soulless eyes dominated front pages in the 1970s. “Some layers are translucent and you would keep peeling because nothing would be revealed, and then every once in a while you’d see something that would lead you to something else.”

“He could be charming and very disarming,” he adds later. “There was something seductive about him, but at the same time he was someone you might be talking to at a cafe, and the next minute you’d look up and he’d be gone. I mean literally disappeared. He was very much that kind of John Le Carre guy.

“We loved him when we had him, but we didn’t often have him. . . . It often seemed he lived his life alone, with the facade of a family. We were the cover.”

Colby’s film revisits Vietnam, of course, where his father first arrived in 1959, family in tow, as deputy chief, later chief, of the CIA station in Saigon. Carl remembers picnics with the family of President Diem, who would be murdered in a 1963 coup backed by the American Embassy. By then, the Colbys had returned to Virginia, where the father was head of Far East operations.

But it was a single — and some say, perfidious — act that defined Colby’s legacy as CIA director: turning over the agency’s darkest secrets to congressional investigating committees.

Colby’s revelations of assassination plots and coup d’etats, some of which he himself called “wrong” and even “deplorable” in the riveting public hearings, made the CIA of that era forever synonymous with dirty tricks.

It also earned him the undying enmity of hawks in the Ford administration, who mocked Colby’s Catholicism. One official said that if Colby felt a need to confess, he should have gone to a priest.

“I often wondered if Bill was not expiating his sins, starting with the Phoenix Program and whatever had gone wrong in it that he felt responsible for,” Brent Scowcroft, Ford’s national security adviser, says in the film. Perhaps, he speculates, the often-absent CIA man felt guilty about not being more attentive to his daughter, Carl’s sister, who died after a long illness in 1973.

Epithets like “choirboy” and “traitor” stuck to Colby and, for Ford, at least, made his further tenure at the CIA impossible. Replacing him with George H.W. Bush was “almost a mercy killing,” Scowcroft says.

“I think he took on the sins of the agency almost as his own, and if he was destined to be jettisoned, to be hung out to dry, so be it,” Carl says over a hamburger in Georgetown.

He didn’t have much of a choice, The Post’s Bob Woodward says in the film.

“All of a sudden, you’re in this political world as CIA director and you have to establish what’s the authority, moral or otherwise. Congress, under Article 1, has this incredible authority over even the CIA. They had the authority. Colby tried to walk a middle line and finally, really acquiesced to the law.”

Decades later, memories of the Colby era still burned like a hot penny in the pockets of many former Republican officials.

James Baker II, who served in the Ford, Reagan and first Bush administrations, traced the intelligence failures of 9/11 to “when CIA Director William Colby was forced to reveal the CIA’s ‘Family Jewels’ and the CIA’s capacity to engage in covert action was destroyed.”

“I was stunned,” Carl wrote for the Huffington Post in September. “My father had been dead for more than five years, and here he was, still part of the debate.”

His father had had enough long before, Carl says. And, like a Roman centurion, he had decided never to grow old and feeble, “to get the AARP card and take the senior discount.”

Was his death a suicide? “I think he just got tired,” Carl says.

To the very end, he was, Carl says, “a Rubik’s Cube of a man.”

Former Post SpyTalk blogger Jeff Stein, a military intelligence case officer in Vietnam, can be reached at SpyTalk@me.com.

The Man Nobody Knew: In Search of My Father, CIA Spymaster William Colby

opens Friday at the Landmark E Street Cinema.

opening Friday at the Landmark E Street theater.