Terrorist and security threats to this country had waned enough by Friday to lower the national terror alert level to "elevated," but the government said it would stay on guard.

Homeland Security Secretary Tom Ridge (search) said Friday that the level would be reduced from orange, or "high" — which it had been at since Dec. 21 — to yellow, or "elevated."

"Today, based on the careful review of intelligence, we have lowered the threat level to yellow," Ridge said during a press briefing. "I know we are all thankful nothing happened. The holidays have passed. They passed safely and without incident."

But he warned that the threat wasn't over.

"Critical resources and locales" will remain on heightened alert, and there still was concern about Al Qaeda's interest in using commercial planes to mount an attack, Ridge said.

"We are still concerned about continued threats, but the threat conditions that we've been following have diminished," he said at a news conference.

He declined to offer specifics. An intelligence official said separately that certain extremists abroad had raised the possibility of attacks in the near term but such attacks no longer were expected.

Certain airports as well as the cities of New York, Washington and Los Angeles will continue to have increased security, said a Department of Homeland Security (search) official who spoke on condition of anonymity.

A few of the many hundreds of dams, bridges and chemical plants that received heightened protection during the higher alert will continue to be treated with extra vigilance, the official said.

Ridge hailed actions taken by federal, state and local officials, among others, to boost security measures while the nation was at orange alert.

"The level of response and resolve to that call to action was exceptional and I might add, virtually unprecedented," Ridge said.

He cited citizens' awareness, various agencies boosting the levels of information-sharing and good communication as some reasons why no attacks occurred. But he encouraged everyone to continue to keep their eyes and ears open.

"Although we have returned to yellow, we have not let our guard down," Ridge said. "And we will maintain particular vigilance around some critical resources and locales."

Earlier Friday, DHS officials told Fox News they weren't naming specific cities or locations that will remain on a heightened state of alert to guard against possible terror attacks.

"We don't want to give Al Qaeda a blueprint for what we're doing," one official said.

Agency officials said Ridge had contacted local officials to maintain security at critical infrastructures like power plants, bridges and nuclear and chemical plants. And airports and airlines would also stay on high alert.

"One of the most persistent and consistent reports that we have from multiple sources is the continued interest by Al Qaeda to use aircraft, but particularly commercial aircraft" for attacks, Ridge said.

He said there was a "growing awareness" among the aviation community of the need to establish security standards in the nation's airports to reduce any future threats.

The alert level was raised from yellow, or "elevated," to orange, or "high," on Dec. 21 after Homeland Security officials said there was a substantial increase in intelligence suggesting that attacks were planned.

Since Sept. 11, the country had never been in more peril of a terrorist attack than it was over the holiday season.

Ridge said "we are in no way apologetic" for raising the alert then, given all of the large gatherings planned around the country.

He stressed that the alert was raised last month because of "the most significant convergence of multiple reporting streams" on terrorist threats since the horrific attacks on U.S. soil over two years ago that left almost 3,000 people dead.

Several international flights were canceled -- some repeatedly -- and military helicopters patrolled during the large celebrations on New Year's Eve in Las Vegas and at Times Square in New York City.

During heightened alerts, airports are asked to restrict parking near terminals if necessary, increase law enforcement patrols of terminals and perimeters and conduct random checks of vehicles on airport roadways.

But U.S. officials told Fox News that the volume of chatter has dropped off in recent days.

Officials deployed nuclear experts to several big cities to hunt for dirty bombs around the time the alert was raised, and initiated a number of other security measures. Experts and government officials have said that Al Qaeda has made it no secret that it is interested in dirty bombs, radiological devices that could cause deaths and mass hysteria if detonated in a crowded area.

During the orange alert period, U.S. officials said they were focused on possible threats to the aviation system, particularly the threat of overseas flights being hijacked and used in Sept. 11-style attacks on American targets.

Fighter jets have been patrolling over some cities, there has been increased security at key government, transportation, energy and nuclear facilities and several international flights to the United States have been canceled, delayed or re-screened after arrival.

Ridge said it may take "weeks or months" to determine for sure if any actions the U.S. and other governments took to thwart any attacks actually deterred potential terrorists.

Experts said that while the news of the alert lowering is good, everyone still needs to be watchful.

The threat has "not gone away but I think it's reduced to a level where they thought they could lower it for the majority of the country," former New York City bomb squad detective Kevin Barry told Fox News.

The government is maintaining a heightened alert for some cities, he said, because "why tell the enemy what you're going to do?"

Up through last weekend, officials had said the alert level would remain at orange for several weeks after the holiday season. The resurfacing of yet another audiotape by Usama bin Laden also led some to believe that an attack somewhere in the world could be imminent.

The terror alert has been raised to orange several times since the color-coded alert system was put in place in early 2002 after the Sept. 11 attacks.

No attacks have taken place on U.S. soil during those orange alerts.

Andy McCarthy, a senior anti-terrorism consultant with The Investigative Project (search), said perhaps one reason there were no attacks in the United States during the holidays is a weakened Al Qaeda network.

U.S. troops and other countries have captured, killed or neutralized "thousands" of terror ring members "who would otherwise be a risk for us," McCarthy said.

"I think that what they're down to now … but it's a very real threat ... are targets of opportunity as opposed to the past when a military battle wasn't being taken to them, they had the luxury to plan."

Fox News' Liza Porteus, Mike Emanuel, Molly Henneberg and The Associated Press contributed to this report.