FORT LEE - U.S. counterterrorism officials are chasing an unconfirmed al-Qaida threat to attack New York or Washington, D.C. in a plot timed to coincide with the tenth anniversary of the Sept. 11 attacks.

Reports say Department of Homeland Security officials obtained information from a credible source who has provided accurate intelligence in the past. They say three men who are allegedly involved in the plot traveled from Pakistan, two of them with U.S. passports, but their identities remain unknown. Officials say they received an indication that al-Qaida planned to use car bombs to blow up bridges or tunnels.

The Port Authority of New York and New Jersey has beefed up security at all of its facilities, including the World Trade Center site. The agency says it has increased police presence and bag checks at Newark Liberty International Airport, John F. Kennedy International Airport and LaGuardia Airport, the Port Authority Bus Terminal and PATH train stations.

The Port Authority also has stepped up vehicle inspections at all of its bridges and tunnels.

Authorities are asking people to report anything they deem suspicious.

AP wire services contributed to this report.