|New York police have amassed a display of force across the city in response to the enhanced 'terror' threat [Reuters]
Police in New York and Washington are on high alert following tip-offs that al-Qaeda could be plotting to detonate a car bomb to coincide with the 10th anniversary of 9/11 attacks on the United States.
Unnamed US officials said the group, which carried out the deadly attacks in 2001 that killed about 3,000 people, might have sent Americans or men carrying US travel documents to launch another attack on Sunday.
One US official said on Saturday that al-Qaeda had dispatched three men, at least two of whom could be US citizens, to detonate a car bomb in New York or Washington.
The attackers were reportedly told to cause as much destruction as they could, but US intelligence officials said they had no evidence there was anyone inside the country tied to the plot.
Word that al-Qaeda had ordered the mission reached officials mid-week.
A CIA informant who has proved reliable in the past approached intelligence officials overseas to say that the men had been ordered by al-Qaeda leader Ayman al-Zawahri "to do harm on US soil".
The tipster said the would-be attackers were of Arab descent and may speak Arabic as well as English.
Counterterrorism officials have been working around the clock to determine whether the threat was accurate, but so far, have been unable to corroborate it, officials said, speaking on condition of anonymity.
Counterterrorism officials were looking for certain names associated with the threat, but it was unclear whether the names were real.
Intelligence analysts have looked at travel patterns and behaviours of people entering the country recently. While they have singled out a few people for additional scrutiny, none has shown any involvement in a plot.
It could be weeks before the intelligence community can say whether the threat against the anniversary was real.
"We're watching," James McJunkin, FBI assistant director in charge of the Washington field office, said on Saturday.
"We expect we're going to get an increase in threats and investigative activity around high-profile dates and events.
Law enforcement around the country had already increased security measures at airports, nuclear plants, train stations and more in the weeks leading up to September 11.
Extra security was put in place to protect the people in New York and Washington, DC, the two cities that took the brunt of the jetliner attacks at the World Trade Center and the Pentagon a decade ago.
Briefed about the threat on Friday morning, President Barack Obama instructed his security team to take "all necessary precautions", the White House said.
Obama still planned to travel to New York on Sunday to mark the 10th anniversary with stops that day at the Pentagon and Shanksville, Pennsylvania.
First 'active plot'
The timing of this particular threat has had officials especially concerned, because it was the first "active plot" that came to light as the country marked the significant anniversary, a moment also significant to al-Qaeda, according to information gleaned in May from Osama bin Laden's compound after he was killed in a US operation.
Britain has warned its citizens travelling to the US that there was a potential for new attacks that could include "places frequented by expatriates and foreign travellers".
US embassies and consulates abroad had also boosted their vigilance in preparation for the anniversary.
In New York on Friday, security worker Eric Martinez wore a pin depicting the twin towers on his lapel as he headed to work in lower Manhattan where he also worked 10 years ago when the towers came down.
"If you're going to be afraid, you're just going to stay home," he said.
Mayor Michael Bloomberg, too, made a point of taking the subway to the City Hall.
At Penn Station in New York, transit authority police carried assault rifles and wore helmets and bullet proof vests as they watched crowds of commuters.
"The NYPD has set up checkpoints on the roads in many locations, and there are also random inspections taking place in the subway system, with bag checks set up at major stations," Al Jazeera's Asad Hashim reported from the city's financial district.
In Washington, Police Chief Cathy Lanier warned that unattended cars parked in suspicious locations or near critical buildings and structures would be towed.