New York ( CNN ) -- Two of the three suspects believed to be involved in a potential 9/11 anniversary plot against New York or Washington are of Arab descent and traveled to the United States last week , according to a U.S. government official . Authorities are operating under the assumption that two of the suspects have arrived on U.S. soil , and have been piecing together clues gleaned from flight logs and manifests , among other sources , said the official , who declined to be named . A third plotter is believed to have been traveling in Europe , though it is not clear whether that person has arrived in the United States , the official said . American spy networks had intercepted communications of an attack from an al Qaeda operative in Pakistan , derived from a source who has provided accurate information in the past , official say . No other corroborating evidence of an attack has been uncovered , but it has prompted intelligence officials to sift through communications from other al Qaeda cells . The information suggests the plan -- thought to involve a vehicle-borne explosive device -- is meant to coincide with the 10th anniversary of the September 11 attacks . The exact type of strike , however , is largley unclear and still remains unconfirmed . U.S. officials rarely speak on the record about intelligence intercepts . And in the days following the raid on Osama bin Laden 's compound , officials indicated al Qaeda had gone to great lengths to avoid having its communications intercepted by the United States . While the precise nature of communication intercepts is rarely discussed , U.S. officials repeatedly have indicated their comfort with sharing the results of those intercepts . Authorities also say they have picked up `` chatter , '' or widely divergent communications , from extremists that suggest the newly tapped al Qaeda head , Ayman al-Zawahiri , is in some way involved in the current plot . Meanwhile , heightened security was visible Saturday in New York and Washington as intelligence officials worked to pin down information about the potential threat . A senior U.S. official said the plot was believed to involve three individuals . It is believed to entail a vehicle bomb , but `` we can not rule out other means , '' the official said . U.S. Vice President Joe Biden also weighed in Friday , saying the information was at least in part gleaned from the May raid against bin Laden in Abottabad , Pakistan . `` This is the first ... credible piece of information we 've gotten , '' Biden told NBC 's `` Today '' show . `` All hands are on deck . '' It is not clear how the bin Laden raid helped authorities connect the dots to the prospect of an anniversary attack , but Biden downplayed the threat of a widely sophisticated plot involving multiple conspirators . He said the administration 's principal concern is a plot from a `` lone actor , not some extremely complicated plan like it took to take down the World Trade ( Center ) towers or the plane in Shanksville ( Pennsylvania ) or the Pentagon . '' He added , `` It does n't mean they could n't happen , but it 's much less likely . The lone actor is the more worrisome thing because there are fewer trails to follow , there are fewer leads to move on . '' On Saturday morning , police officers stopped and searched box trucks as they approached the George Washington Bridge , which links New Jersey with New York , as well as the Queens-Midtown Tunnel , the Holland Tunnel and the Verrazano-Narrows Bridge , connecting the boroughs of Staten Island and Brooklyn . Police also manned checkpoints in Midtown and downtown Manhattan , where they examined vehicles , particularly vans and trucks , for possible bombs . New York police also are sweeping parking garages for explosives and using digital license plate readers to check for stolen vehicles . New York Gov . Andrew Cuomo has directed state police and asked National Guardsmen to help secure areas during the anniversary weekend , bolstering an already heightened level of security . `` We already had a great security plan in place , and in an abundance of caution , we 're deploying more resources , '' he told reporters . `` We should n't allow this threat to diminish the importance of the 9/11 anniversary , because that would be doing just what the terrorists want us to do . '' A senior U.S. administration official said Friday that it is the `` origin '' of the intelligence information that is causing particular concern . U.S. officials believe that operatives came out of the tribal Pakistan-Afghanistan border region -- a volatile semi-lawless area that is home to extremist groups -- and that they are part of al Qaeda `` central . '' The official also noted a possibility that Pakistan-based groups such as Lashkar e-Tayyiba or Tehrik-i-Taliban Pakistan could be involved . A senior White House official said President Barack Obama had been updated about the potential threat after his speech to a joint session of Congress on Thursday night , and White House counterterrorism adviser John Brennan briefed him again Friday . No changes have been made to the president 's schedule for the weekend . New York City Mayor Michael Bloomberg told reporters Thursday night that while additional police will be deployed around the city , `` There 's no reason for any of the rest of us to change ... our daily routines . '' A joint intelligence bulletin issued by the FBI and the Department of Homeland Security on Thursday indicated other methods than a truck or car bomb could be used , including attacks with small arms , homemade explosives and poisons . Al Qaeda probably would provide its operatives with enough autonomy to select the particular target and method of attack , the bulletin said , and an attack may involve operatives carrying U.S. documentation . Intelligence officials believe al Qaeda `` likely maintained an interest since at least February 2010 in conducting large attacks in the Homeland timed to coincide with symbolic dates , to include the 10-year anniversary of the 9/11 terrorist attacks , '' according to the intelligence bulletin . Bin Laden 's death and the `` removal '' of senior al Qaeda figures since then could add to the organization 's desire to stage an attack on a symbolic date such as September 11 , the bulletin said . In Washington , local officials said the city has a robust plan in place and has been on heightened alert since September 1 . `` I want to urge all of our citizens ... to remain calm and let our law enforcement professionals do their job . At the same time , we ask people to remain vigilant , '' Mayor Vincent Gray said . `` If you see something , say something . '' Unattended vehicles around key infrastructure will be towed , Washington Metropolitan Police Chief Cathy Lanier said . Federal officials said they are taking the threat seriously , while trying to temper the news by saying such threats are commonplace in connection with key dates . Rep. Peter King , R-New York , chairman of the House Homeland Security Committee , said specific details were revealed to lawmakers about the threat . `` Many agencies are looking at this from every possible angle , '' he said . But it is not known yet if the threat is real , he said , adding , `` I would tell people now to go about their lives . There 's no need to panic . '' CNN 's Mike M. Ahlers , Lesa Jansen , Allan Chernoff , Thomas Evans , Terry Frieden , Jessica Yellin , John King , Jim Barnett , Pam Benson , Fran Townsend and Ross Levitt contributed to this report .
NEW : Two Americans of Arab descent are believed to be involved in 9/11 plot . NEW : Authorities are piecing together clues gleaned from flight logs and manifests . NEW : A third plotter is believed to be have traveling in Europe . Gov . Andrew Cuomo directs state police to help with security at major transport hubs .