The killing of the American cleric Anwar al-Awlaki will instill a "sense of doom" in al-Qaida's surviving leaders, a senior American military official told The New York Times.
The official, who was not named by the paper, said al-Awlaki's death was "critically important" and would send an important message to everyone in the terrorist network.
He spoke as New York's police commissioner Ray Kelly said the city's force was on high alert in case al-Awlaki's followers tried to stage a retaliatory attack.
"We know al-Awlaki had followers in the United States including New York City, and for that reason we remain alert to the possibility that someone might want to avenge his death," Kelly said in a statement.
"He was a powerful recruiter of terrorists in the United States," he said.
The Times reported that the airstrike that killed al-Awlaki and fellow U.S. citizen Samir Khan, 25, was the culmination of a two-year manhunt.
It said there had been some close calls, including a U.S. drone strike in May and a previously unreported operation led by Yemen's counterterrorism commandos.
The elite unit and army soldiers surrounded a village and tried to persuade local leaders to hand al-Awlaki over, a member of the unit told the paper.
"We stayed a whole week, but the villagers were supporting him," the counterterrorism officer, who is not authorized to speak on the record," told the Times. "The local people began firing on us, and we fired back, and while it was happening, they helped him to escape."Story: Radical American cleric killed in Yemen, officials say
The senior U.S. military official told the Times that the ultimate success of the attempts to kill al-Awlaki would limit the capabilities of al-Qaida.
"It sets a sense of doom for the rest of them. Getting Awlaki, given his tight operational security, increases the sense of fear. It's hard for them to attack when they're trying to protect their own back side," he said, according to the paper.Can U.S. legally kill a citizen without due process?
However one Islamist figure, Anjem Choudhry, an Islamic scholar in London, told the Times that al-Awlaki's death would spread his fame.
"The death of Sheik Anwar al-Awlaki will merely motivate the Muslim youth to struggle harder against the enemies of Islam and Muslims. I would say his death has made him more popular," he said.Story: Plenty of al-Qaida targets remain after Osama bin Laden's death
However, at the Washington-area mosque where al-Awlaki preached a decade ago, there were few tears over his death.
Most worshippers at Dar al-Hijrah for Friday services said they were glad that al-Awlaki was gone — that he besmirched not only their mosque but all of Islam by calling for the deaths of innocent Americans.
'Good for humans'
Khalid Abutaa was among those happy to hear the news that al-Awlaki was dead.
"It's good. It's good for Muslims. It's good for humans," said Abutaa, a retired chef. "In our religion, we're not supposed to kill nobody."
Others rejected both al-Awlaki's calls for violence against Americans and the U.S. airstrike that killed him in Yemen early Friday, saying he hadn't even been charged with a crime.Story: Radical cleric influenced many plots, US says
Leaders of the mosque issued a statement saying that although al-Awlaki "encouraged impressionable American-Muslims to attack their own country," they deplored "extra-judicial assassination" and believed the drone attack "sends the wrong message to law-abiding people around the world."
Hassan Mohamed, 62, said no allegations against al-Awlaki had been proven.
"I don't know why he should be killed," he said. "I don't approve of my government going around the world to kill innocent people."
But a small few were unrepentant in their support of al-Awlaki, though most were unwilling have their names attached to their views.
The mosque endures some level of hostility from the general public.
'Got your little buddy'
On Friday morning, as a crowd started to gather outside the mosque before midday services, a bicyclist rode by and shouted, "Yeah, they got your little buddy, didn't they?" then spit on the ground before pedaling off.
Jibril Hough, spokesman for the Islamic Center of Charlotte, N.C., told The Associated Press that leading members of the local Muslim community had tried to persuade Khan to abandon his Islamist beliefs and renounce violence.Video: Second American killed in al-Awlaki strike
Hough said he called Khan's father in 2008 after Samir's ideology became known and arranged a counseling session, "an intervention of sorts."
There were two meetings in Hough's home over the course of a month involving Khan, his father and a handful of other respected members of the Muslim community, Hough said. Each lasted several hours.
"He was very respectful — kind of quiet. He didn't give us a big argument. There was a time or two he tried to state his case. He was pretty much respectful of the circle we'd set up, and he listened," Hough said.
The few words Khan tried to offer then involved defending his view supporting the killing of innocent people, Hough said.
'I am a traitor to America'
Khan cut off ties with his family when he went to Yemen in 2009, Hough said.
He had spent several years in the U.S. editing a web site praising al-Qaida leaders.
"I was quite open about my beliefs online and it didn't take a rocket scientist to figure out that I was al Qaeda to the core," Khan wrote in the fall 2010 issue of Inspire magazine, an online publication. "I am a traitor to America because my religion requires me to be one."
Khan's life in Yemen involved helping produce the irreverent, graphics-heavy Internet magazine aimed at recruiting young Muslims to the jihadi cause with articles such as, "Make a Bomb in the Kitchen of Your Mom."
Hough said Khan's family was in mourning Friday and did not want to talk about their son, who was 25.
"Even though we don't believe in the path he was going or the way he was thinking, he was still a human being, still a human life, and he was still someone's son," Hough said.
Reuters and The Associated Press contributed to this report.