WASHINGTON — Just days before the 10th anniversary of the Sept. 11 attacks, U.S. counterterrorism officials were chasing a credible but unconfirmed al-Qaida threat to use a car bomb on bridges or tunnels in New York City or Washington.
It was the first known "active plot" timed to coincide with the somber commemoration of the terror group's 9/11 attacks a decade ago that killed nearly 3,000 people.
Counterterrorism officials were investigating the threat early Friday, as police in New York and Washington said they would increase their already stepped-up staffing levels in light of the recent intelligence.
An Obama administration official told NBC News Thursday that the threat "so far" was a 5 to 6 on a scale of 10.
Officials emphasized that there was a great deal of uncertainty about the threat and said they were acknowledging it out of an abundance of caution.
Reuters quoted a U.S. official as saying al-Qaida leader Ayman al-Zawahri may be linked to the threat, but that's highly unlikely, senior officials told NBC News.
"Bin Laden was more involved in al-Qaida operations" than al-Zawahiri has been since he took over as al-Qaida's No. 1, one of the officials said. "He's too busy trying to stay alive."
Still, the White House does believe some element of al-Qaida is behind the threat — just not al-Zawahiri himself.
Barack Obama received another briefing on the potential threat from National Security Adviser Tom Donilon, Chief Counterterrorism Advisor John Brennan and others Friday morning.
Jay Carney, White House press secretary, told reporters that the president ordered the national security team to double their efforts and take "all necessary precautions," NBC News reported.Video: NYC, DC heighten security after ‘credible’ threat (on this page)
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Law enforcement officials have been told to be on the lookout for three men "of Middle Eastern descent" and potentially traveling in a van, NBC News reported.
They may have recently entered the country, based on the detailed information received by the U.S. intelligence community late Wednesday, officials said.
The intelligence suggested that al-Qaida planned to car bomb one of the two cities that were hit 10 years ago.
The information had some specificity and the source has been credible in the past, officials said.
"There is specific, credible but unconfirmed threat information," the head of the FBI's New York division, Janice Fedarcyk, said. "As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days."
The nation's terror alert level has not changed, but raising it was under consideration.Should you worry about traveling on 9/11?
Officials have been concerned that terrorists would see the 9/11 anniversary as an opportunity to retaliate against the U.S. for killing bin Laden in a military raid in May.
However, New York City Mayor Michael Bloomberg rode the subway Friday morning to assure commuters that the city was fully prepared for a potential terror threat.
"We don't want al-Qaida or any other organization ... to take away the freedoms without firing a shot," he said, getting off at the City Hall stop in lower Manhattan near the Brooklyn Bridge.
Bloomberg urged New Yorkers to just "go back to work. And leave it to the professionals."
'Wreak violence and evil'
Secretary of State Hillary Rodham Clinton, in a speech called "A Smart Power Approach to Counterterrorism" given at the John Jay College of Criminal Justice Friday morning, said the threat of terrorist acts in the United States "should not surprise any of us," saying it served as a "reminder of the stakes in our struggle against violent extremism."
"We are taking this threat seriously, federal, state and local authorities are taking all steps to address it," she said. "And of course making it public as it was done yesterday is intended to enlist the millions and millions of New Yorkers and Americans to be the eyes and ears of vigilance."Video: New 9/11 tapes show confusion and disbelief
"Of course, people should proceed with their lives and do what they would do ordinarily, but ... (also) be part of this great network of unity and support against those who would wreak violence and evil on innocent people," she added.
Clinton added that America would not "shy away from using military force" if necessary against terrorist groups such as al-Qaida.
Terrorism suspects would be detained "humanely" and "when we do strike," the U.S. would seek to protect innocent civilians.Video: Biden: Americans must ‘be vigilant’ (on this page)
Gail Murray, an administrative assistant who works in Manhattan, took the security threat in stride as she listened to Long Island Rail Road announcements aboard a train heading from Queens Village to Penn Station.
"I thought, 'Here we go again,'" she said. "That's all just part of living in New York City."
She said whether or not she was worried, she would have to take mass transit.
"I don't have the luxury of working from home," she said.
Authorities were stopping vehicles at the 59th Street bridge, which connects Manhattan to Queens, causing a major backup.
Helmets, assault rifles
At Penn Station, transit police in helmets and bulletproof vests and carrying assault rifles watched the crowds of commuters.
Officials were swabbing passengers' bags near an escalator to the train platforms, and police searched the bags of passengers at the entrance to a subway station. National Guard troops in camouflaged fatigues moved among the throng, eyeing packages.
Roseanne Lee, 64, said her taxi was stopped twice at police checkpoints on its way from the Upper East Side to Penn Station.Find out where 9/11's Bob Beckwith is now
Police looked in the windows of the cab but did not question her or the driver, she said. At one checkpoint, police were searching a moving van, she said.
The delays turned a 15-minute ride into a 35-minute one and cost her $21 instead of the usual $12.
"But I don't care," Lee said. "It's better to be safe. You can't stop doing what you're doing because of these threats, you just have to be careful."
Police tours were extended, effectively increasing the strength of the patrol force, and the department prepared to respond to an increase in calls of suspicious packages. They also added more police vehicles with license plate readers.
"There will be increased focus on tunnels and bridges and infrastructure in general, as well as landmark locations, houses of worship and government buildings," Police Commissioner Raymond Kelly said.
The Port Authority of New York and New Jersey, which owns the trade center site, was also at a heightened state of alert Friday.
In Washington, law enforcement officials said they were working 12-hour days indefinitely, and Police Chief Cathy Lanier said unattended cars parked in unusual locations risked being towed.
She said that every one of her 3,800 officers would work at some point during remembrances.
"You'll see mass transit, you'll see restaurants, hotels, sporting events — any place where there's a crowd, we're going to have an increased presence," Lanier said.Video: Giuliani: ‘I wish I had anticipated’ 9/11 (on this page)
The U.S. Park Police, which has jurisdiction over D.C. landmarks including the National Mall and the Washington Monument, was also on alert.
"We're well aware that this anniversary does bring with it a lot of emotion and concerns, but we're certainly addressing it," said Park Police spokesman David Schlosser.
Obama is scheduled to mark the 9/11 anniversary with stops at New York's ground zero, the Pentagon and Shanksville, Pa. He also will deliver remarks Sunday night at a memorial concert at the Kennedy Center in Washington.
President George W. Bush also plans to attend Sunday's ground zero event.
Kelly, the NYPD's commissioner, said that ground zero would be surrounded by a frozen zone on Sunday. Thousands of cops will be on duty, some carrying sniper rifles. Hundreds of surveillance cameras will monitor the site.
Kelly said quick-reaction forces will be assigned outside lower Manhattan.Video: George W. Bush reflects on 9/11, ‘fog of war’ (on this page)
Police scuba divers will also examine the USS New York, made of World Trade Center steel, at its West 48th Street dock, Kelly added.
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Already responding to rampaging rivers , Pennsylvania emergency workers say they are preparing for whatever security threats Sept. 11 events may bring.
National Park Service’s Flight 93 National Memorial in Stonycreek Township near Shanksville will be unveiled Saturday during events that are expected to draw 10,000 people, including Vice President Joe Biden.Video: Inside the ‘bigger, better' World Trade Center (on this page)
Cory Angell, a PEMA spokesman, told radio station WHYY the center nearest to the site of the Flight 93 memorial in Somerset County will be activated at a Level 2 — meaning key emergency staffers from different state departments will be on duty.
"In the western part of Pittsburgh, of course — Flight 93 and the event that they're having in memory of those lost on 9/11 — being a significant event, that EOC will be activated at a Level 2, as well," he said.
The Flight 93 memorial's website notes that security checkpoints will be located at ceremony entrances, chairs will be permitted but not in bags; coolers are banned.
Among major commemorations:
8:30 a.m.-5:30 p.m. - Bill Clinton will speak at this week's dedication of the Flight 93 National Memorial in western Pennsylvania. The former president will join former President George W. Bush, Vice President Joe Biden, Speaker of the House John Boehner and Flight 93 family members at Saturday's event (which runs 10 a.m. - 5:30 p.m.). Sunday is the 10th anniversary commemoration of the Sept. 11 terrorist attacks. Events will take place 8:30-11 a.m. and 1:50-5 p.m. The events are open to the public, and security is expected to be tight.
8:30 a.m.-1 p.m.: The National September 11 Memorial in New York will be dedicated during a ceremony for victims' families. President Barack Obama and former President George W. Bush plan to attend. The memorial opens to the public the following day.
9:30 a.m.: The Pentagon Memorial will have an invite-only ceremony for families of 9/11 victims at 9:30 am. The Pentagon Memorial will re-open to the general public following the ceremony.
1-4 p.m.: HandsOn Greater DC Cares and Serve DC - the Mayor's Office of Volunteerism will commemorate the 10th anniversary of 9/11 by hosting Tribute to Service – Honoring the Victims, Survivors, and Heroes of 9/11 at Freedom Plaza in downtown Washington, D.C. from 1-4 p.m.
NBC News, msnbc.com staff and The Associated Press contributed to this report.