SITE Intelligence Group/AP Photo

“This is a great day for America,” says a senior administration official, pleased with the news that terrorist leader Anwar al Awlaki was killed today.

The Yemeni government today announced that an operation launched shortly before 10 a.m. in Yemen targeted and killed the New Mexico-born cleric, a missile hitting him approximately five miles from the town of Khashef, 87 miles east of the capital Sana’a.

Senior administration officials say that the U.S. has been targeting Awlaki for months, though in recent weeks officials were able to pin down his location.

“They were waiting for the right opportunity to get him away from any civilians,” a senior administration official tells ABC News.

In fact, there was a flurry of activity on the 10th anniversary of 9/11.

As President Obama shuttled between Shanksville, Penn., New York, NY, and the Pentagon, officials “thought they had a good opportunity to hit him,” the official says. “We waited, but it never materialized.”

A senior White House official says Awlaki was “very operational, every day he was plotting, he had very unique skills, and it’s good to get him in Yemen where AQAP” – al Qaeda in the Arabian Peninsula – “is planting the flag.”

Out of deference to the Yemeni government, U.S. officials have been tight-lipped about the precise role the U.S. played, though the Yemeni military does not possess the kind of predator drone said to have killed Awlaki.

The senior White House official tells ABC News: “we’ve had a very intense focus on him. And once we decide to put resources on one of these guys, I’d hate to be one of them. Unless you’re literally in a cave, which is where we think (Ayman al-) Zawahiri is, any visibility — and to be operational you have to have that, so one phone call, one email, one car ride — and you’re found.”

In early February 2010 – fewer than two months after failed Christmas Day underwear bomber Umar Faruq Abdulmuttalab, who had links to Awlaki, tried to strike – the National Security Staff put out an early directive saying Awlaki is a valid target for killing. Since the cleric was an American citizen, administration lawyers vetted the argument, ultimately concluding that Awlaki was a viable target since he cannot be captured.

Since then, Awlaki has been tied to Fort Hood shooter Nidal Hasan of November 2009. Failed Times Square bomber Faisal Shahzad, who tried to strike in May 2010, said he was inspired by Awlaki.

In June 2010, then-CIA director Leon Panetta told me on THIS WEEK “Awlaki is a terrorist, and yes, he’s a U.S. citizen, but he is first and foremost a terrorist and we’re going to treat him like a terrorist. We don’t have an assassination list, but I can tell you this, we have a terrorist list and he’s on it.”