WASHINGTON - For an unprecedented presidential inauguration, an extraordinary network of security precautions has been created.
From road closures in downtown Washington to a ban on most traffic over the Potomac River, today's inauguration of Barack Obama will employ a mix of visible and clandestine protections apparently larger in scope than anything the nation's capital has seen.
Obama and his family have received threats in the past. But security officials say they are driven primarily by the size of the crowds - perhaps as many as 2 million - expected to gather as the first African-American president takes the oath of office.
As the Obamas and other VIPs attend today's inauguration and parade, and the balls and galas that follow, they will be safeguarded with layers of security agents, armored limousines, clear bulletproof shields, rooftop snipers and other measures that officials won't discuss in detail.
Officials working under the U.S. Secret Service have coordinated police and security specialists from 58 government agencies focused on keeping VIPs and visitors safe from risks that include homemade bombs, the deadly terrorist devices known from Iraq as improvised explosive devices, or IEDs.
Significant steps have been taken to thwart more general potential terrorist incidents.
Roads and bridges into the city from Virginia will be closed and traffic restricted from the Maryland side. The Coast Guard will cordon off the Potomac River closest to Washington, banning even kayaks.
Overhead, the airspace out to 30 miles from the White House and up to 18,000 feet will be tightly restricted, with even balloons and model planes and rockets outlawed. Air Force F-15 and F-16 fighters will circle on combat air patrols and may be authorized to "use deadly force," according to a notice posted by the Federal Aviation Administration.
Several hundred teams of dogs that can sniff out traces of explosives or hazardous chemicals, normally stationed at airports, are being deployed in Washington.
Among others assisting with security are 1,200 Maryland National Guard soldiers, part of a 10,000-strong National Guard force drawn from Maryland, Virginia and other states as far away as Iowa. Most of the Maryland troops will team up on the National Mall with U.S. Park Police.
The Maryland Guard is also sending its Civil Support Team, 22 soldiers highly trained in chemical and biological agent detection and identification. Its team members will link up with Washington fire and police experts and fan out across the Mall, said Lt. Col. Paul Kastner, the unit's commander. He will have its high-tech mobile lab standing by to analyze any contaminants that are found.
But U.S. officials and security experts say the IED remains a key focus.
IEDs are "far and away the greatest threat" to Americans even during normal times, outgoing Homeland Security Secretary Michael Chertoff said last summer. Such devices, even so-called "dirty" bombs that spread radioactive material, can be made of easily obtained ingredients by someone with no special skills, he said.
It is not the complex, sensational terrorist attack that officials most fear - another Sept. 11 - but rather the simple, easy-to-carry device that is the most difficult to detect.
"We're talking about an attempt by somebody to exploit the presence of the world's news media and the symbolic importance of this event to attract attention to their cause," said Brian Jenkins, a terrorism expert at the RAND Corp., a Santa Monica, Calif., think tank.
"Preventing some kind of attack during the inauguration, given the size of Washington and the millions of expected visitors, is extremely difficult."
That is especially true if the terrorist is aimed not at well-protected VIPs, but at crowds packed into a subway car or Metro station - with the intent of causing as much panic and disruption as possible.
"A big IED is easier to protect against" because the amount of materiel required can easily be discovered during routine sweeps of streets, vehicles and sewers, said Mike Weber, a retired military explosives expert and VIP protection agent. "The threat today is suicide bombers, and the only real protection is if you get lucky."
"The low-tech event is what concerns me, one that doesn't require a lot of expertise, planning or esoteric materiel," said David Cid, a retired FBI counterterrorism expert who has led security planning for World Series and Olympics.
"The threat is real and tangible, and I am confident we are doing everything we can do make it a secure event. But terrorists only have to get lucky once."
The problem is the expected size of the crowd.
About 240,000 people have tickets to tomorrow's swearing-in ceremony at the Capitol and 5,000 others hold tickets to bleacher seats along the parade route. Tens of thousands more are expected to press as close as possible. Security officials suggested that when the number of Pennsylvania Avenue onlookers reaches 300,000, they will close access to the parade route, where the heavily fortified new Cadillac limousine that will carry Obama will make its debut. Large crowds are expected to throng the streets and subway stations across the city for the 10 official balls and dozens of other celebrations that will follow.
In an effort to deter suicide bombers, police will prohibit pedestrian backpacks, packages, shopping bags, bicycles, insulated flasks and even purses larger than 6-by-8-by-4 inches. At some points, crowds will be forced through electronic screening; at other points, screening will be random. Everyone will be subject to further random searches and screenings.
Other security guards trained in behavior modeling will be scanning the crowd looking for people who seem out of place, who are acting furtively or seem distracted, officials said.
But experts say it is impossible to guarantee that no one is trying to smuggle in a small bomb or a grenade, especially with people swaddled against the cold in heavy coats.
That means security officials are focusing their resources on the areas of most peril. In Washington, as elsewhere, that means the subway.
"When terrorists want to kill in quantity, public transportation has been their killing fields," said Jenkins, referring to terrorist attacks in the past few years in London, Madrid and elsewhere.
Protective measures, including random searches and additional security guards, will be focused on the most vulnerable portions of the Metro system, officials said. Metro has added 200 security guards to its force and has accepted offers of help from transit police from Philadelphia and elsewhere.
If prevention efforts fail, help is standing by, including hundreds of firefighters, rescue workers and others. A team of Marines - trained in extracting victims from rubble, emergency medical care and decontaminating victims after chemical or biological attack - is poised to respond by high-speed hovercraft from Stump Neck, downriver from Washington, a Marine spokesman said.
But even with all this potential trouble, terrorism experts advise people to come and enjoy themselves.
RAND's Jenkins says it's simple math.
A terrorist bomb in a subway historically kills about 20 people, he said. Given 1 million visitors to the inauguration, that's a chance of dying of one in 50,000. The odds of being killed in a car accident are one in 8,000 or being murdered are one in 18,000, according to Jenkins.
Given that most murders are committed by someone the victim knows or is related to, he said, "You are actually statistically safer going to the inaugural than to a family reunion."