Updated 10:35 p.m. ET
U.S. officials said Thursday they were investigating a credible but unconfirmed threat that al Qaeda was planning to use a car bomb to target bridges or tunnels in New York City or Washington to coincide with the 10th anniversary of the Sept. 11 attacks, the first tip of an "active plot" around that date.
Law enforcement officials were investigating three people who recently entered the U.S. The threat was received by the U.S. intelligence community late Wednesday night, officials said.
"There is specific, credible but unconfirmed threat information," said Janice Fedarcyk, the assistant director in charge of the FBI's New York division. "As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days."
According to CBS News correspondent Bob Orr, this threat is "specific and credible but unconfirmed," meaning that intelligence agencies across the government has picked up a threat stream from overseas, and that there is a full court press by the FBI, CIA and others to refute or corroborate the information.
Sources told CBS News that the threat specifically has come from Pakistan, which is home to terror groups, including the Pakistani Taliban, the organization behind the failed Times Square bombing attempt.
A federal law enforcement source also told CBS News that operatives are acting on Osama bin Laden's wishes to time an attack to the anniversary.
While the report is unconfirmed, a number of officials say that there are enough elements lined up to have them concerned about this threat stream more so than the others they have seen in recent days.
At a news conference Thursday night, New York City Mayor Michael Bloomberg says a terror threat against the city is credible but not corroborated. He said at police headquarters the New York Police Department is deploying additional resources around city to keep residents safe.
Bloomberg also said that there was no reason for people to change their daily routines. He mentioned: "If you see something, say something."
NYPD Commissioner Raymond Kelly said he's increasing patrols and the towing of illegally parked cars. He said there will be more bomb-sniffing dogs on the streets and an increased focus on bridges, tunnels and other infrastructure.
A Federal source told CBS News that FBI and intelligence authorities are currently tracking two distinct threat streams -- one involving a car bombing threat in New York or Washington and a separate threat against bridges and tunnels in New York. It is unknown if there is a correlation between the two threats.
A White House official told CBS News White House correspondent Mark Knoller that President Obama was briefed about the threat Thursday morning and had been updated all day. "The United States government has already significantly enhanced its security posture in advance of the 9/11 anniversary to protect the country against possible terrorist threats," said the official. "Nevertheless, the President directed the counterterrorism community to redouble its efforts in response to this credible but unconfirmed information."
White House officials said there were no plans to change Obama's travel schedule on Sunday in light of the threat. The president is scheduled to mark the 9/11 anniversary with stops at New York's ground zero, the Pentagon and Shanksville, Pa. He will also deliver remarks Sunday night at a memorial concert at the Kennedy Center in Washington.
"It's accurate that there is specific, credible but unconfirmed threat information," Homeland Security Department spokesman Matt Chandler said in a statement. "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 Thursday night.
Earlier, Homeland Security Secretary Janet Napolitano said there was no information about a specific threat around the 10th anniversary of the Sept. 11 terrorist attacks but there is "a lot of chatter."
Napolitano told reporters Thursday that "in the intel world there is lots of chatter and we are taking it very seriously."
CBS News reports that the secretary said more federal air marshals would be deployed for the anniversary but wouldn't specify other measures being taken by Homeland Security and other agencies other than that they were "preparing accordingly."
Napolitano said Homeland Security would warn the public if a threat arose.
Asked about the threat of "lone wolves," Napolitano said it is a "key concern" that "more and more Americans in-country are being radicalized to the point of violence" and that Internet recruiting by those like U.S.-born cleric Anwar al-Awlaki is having an impact, CBS News reports.
Speaking generally and not in reference to the 9/11 anniversary, Napolitano said a 9/11-style plot would be "much more difficult" to execute now than 10 years ago because of the multiple security layers now in place, better intelligence gathering and better intelligence sharing, among other improvements, CBS News reports.
In New York, despite no specific terrorism threats against the city, thousands of extra police officers will be deployed, officials said Wednesday.
Police Commissioner Raymond Kelly, in remarks at a security conference in midtown Manhattan, cited evidence found in Osama bin Laden's compound after his death suggesting he hoped to strike on the anniversary.
"For that reason alone, we need to take precautions as if an actual plot is under way," Kelly said.
The New York Police Department plans to form a zone around the World Trade Center for a Sunday observance that President Obama and former President George W. Bush plan to attend. Along with extra officers -- the department won't reveal an exact number -- the security also will include hundreds of surveillance cameras trained on the site, Kelly said.
The World Trade Center gathering also will showcase a new police command of 212 officers dedicated to protecting the site. The officers have received special counterterrorism training, including on how to spot people conducting reconnaissance and how to respond to suicide or truck bombers.
The command conducted drills this week on evacuating the National September 11 Memorial & Museum and its adjacent plaza, which opens Monday.
Sunday's observance is one of more than 30 commemorative events scheduled for the week that the department is laboring to protect, Kelly said.
"We don't presume that ground zero alone is a potential target," the commissioner said. "That would be shortsighted."
The department also will deploy so-called quick-strike reaction forces to respond to potential threats outside lower Manhattan, Kelly said. The teams include officers in heavy armor, bomb squad technicians and hostage negotiators who will have highway patrol escorts on standby if needed.
New Yorkers will see more officers in the subways as well. Many will concentrate on busy transportation hubs such as Grand Central Terminal, Pennsylvania Station and the Herald Square subway station.
There are other security concerns: Extra patrols also will be deployed in Queens for Sunday's U.S. Open tennis finale. And the department is gearing up for next week's U.N. General Assembly, working with the Secret Service to protect 130 heads of state and coordinate 220 motorcades, Kelly said.
"In other words, we have a lot on our plate," he said. "But we have the talent and the resources to deal with it."