Homeland Security officials said Thursday evening they were investigating a detailed al-Qaida car bomb plot aimed at bridges or tunnels in New York or Washington. Unattended cars parked near critical infrastructure will be towed, officials in both cities said.
"We don't take this threat lightly. We take it very seriously, but we are ready for it," said James McJunkin, assistant director in charge of the FBI's Washington field office.
McJunkin said his agents aren't seeking out any particular individuals but were pursuing tips and leads and analyzing patterns about unusual behavior.
"There's no named individual," he said.
District of Columbia Police Chief Cathy Lanier said officers would be working 12 hour shifts for the near future. She said in a written statement that the scheduling changes were "part of our plan" and that "maintaining a certain sense of unpredictability is essential to the success of any security plan."
The chief told AP earlier in the week that every sworn officer would be working throughout the weekend. The department has roughly 3,800 officers.
"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 told The Associated Press.
All Sept. 11 commemorative events will go on as planned, Lanier said, though a Sunday event featuring President Barack Obama was moved from the Washington National Cathedral to the Kennedy Center following the collapse of a massive crane brought in to repair damage from the Aug. 23 earthquake.
In New York, the police department is deploying thousands of extra officers and will form a zone around the World Trade Center for a Sunday observance that President Barack Obama and former President George W. Bush plan to attend. There'll also be hundreds of surveillance cameras monitoring the site. Police in cities including Houston and Phoenix will also be doing more patrols.
Amtrak says it will do more passenger and baggage screenings, deploy more K-9 explosive detection teams and have a stronger police presence at stations and on trains. Metro Transit in Washington and its neighboring suburbs will also have additional patrols and K-9 units but has not received any specific or credible threat, said spokesman Dan Stessel.
In Washington, people walking or traveling near the U.S. Capitol will see more uniformed and plainclothes officers, additional police cruisers and may notice a bomb squad or SWAT team in areas of Capitol Hill where they wouldn't ordinarily be seen, said Sgt. Kimberly Schneider, a U.S. Capitol Police spokeswoman. The U.S. Park Police, which has jurisdiction over D.C. landmarks including the National Mall and the Washington Monument, is 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.
Dozens of travelers were lined up at Union Station on Thursday night to take Amtrak and commuter trains home from Washington. There was no visible increase in security for those boarding trains after news of a security threat, though some passengers said they had noticed police officers and dogs earlier.
Matt Harris, 27, of Baltimore said he had noticed more uniformed officers during his daily commute by train to law school at Georgetown University. By 8:45 p.m., he had seen the news of a security threat against the U.S. while browsing the Internet on his laptop. He said he wasn't worried.
"It's kind of become routine to see more police officers and dogs," he said. "You kind of feel that things are being taken care of."
The terrorist attacks have particular resonance in Washington, as one of the hijacked planes departed from Washington Dulles International Airport and soon after crashed into the Pentagon.
Lanier said the security in place this weekend has been necessary for at least the last 10 years. In D.C., suspicious packages and vehicles are reported and searched constantly throughout the city -- evidence of a heightened public awareness produced at least in part by the terrorist attacks.
"It's not a good idea for everybody to relax after this anniversary is over. This is a way of life for us now," Lanier said.