WASHINGTON — As the multitudes arrive for the historic inauguration of Barack Obama, the most high-tech security bubble ever created is in place to protect the incoming president from any foreseeable act of God, nature or man.
But authorities say they still dread the "X factor" -- intangibles that they cannot control and that could upend their most carefully laid plans by panicking the immense crowd.
At the top of that list, they say, is the lone-wolf individual or small group capable of slipping through the intelligence and security net. A burst of gunfire or an explosion, they know, could cause significant casualties or pandemonium.
That is considered extremely unlikely. Law enforcement and intelligence officials say they have seen nothing to suggest the president-elect or his inauguration are being targeted.
Still, they say, no one can know for sure. And the inauguration of the first black president -- coming in the first White House transition since the Sept. 11 attacks and about two months after the terrorist strike in Mumbai -- poses special concerns, U.S. law enforcement and security officials say.
"We can prepare and we can prepare, but there are always variables," said one senior FBI official involved in the planning. "It only takes one person to come in and cause havoc."
Many disgruntled individuals fly under the radar, officials say, keeping violent thoughts and plans to themselves and out of Internet chat rooms that have been monitored for years by undercover agents.
And they easily could hide among millions of visitors expected to overload transit systems and wedge into every nook and cranny of indoor and outdoor space, straining crowd-control measures.
By most accounts, the Secret Service has done everything possible to safeguard not only the president-elect and his entourage, but the parade route, the National Mall and other locations that will be part of Obama's short but symbolic path to the swearing-in.
For instance, Obama will be riding in a new limousine -- nicknamed "The Beast" -- considered the most secure ever, virtually impervious to chemical and biological attacks and rocket-propelled grenades.
And as in all major events, the Secret Service has spent months working with dozens of local, state and federal agencies on security, crowd control and logistical support.
Thousands of extra police officers and military troops are being brought in from around the country, and measures to protect against chemical and biological attack will be in place, along with decontamination tents.
At least 150 multiagency "intel teams" will deploy throughout the region so that undercover FBI agents and other behavior analysis specialists can look for trouble. Of particular interest: individuals or small groups of men with backpacks lurking in large crowds, or entering the Metro from distant suburban stations -- a pattern similar to the deadly attacks in London's subways in 2005.
In some areas, Washington will look like an occupied city. Sharpshooters will be on virtually every building. Law enforcement and intelligence nerve centers and mobile command posts are sprouting. The FBI is deploying a scary-looking armored assault vehicle and a weapons-of-mass-destruction response truck.
The military, supporting civilian authorities, is using sophisticated surveillance systems developed for the Iraq and Afghanistan wars to monitor the Mall. A P-3 Reconnaissance plane will fly above the Mall to collect information that can instantaneously be transmitted to the ground in the event of a threat. The high-tech system is known as "Rover."
The Pentagon also will station cameras and other detectors on buildings around the Mall on Inauguration Day to ensure a constant picture, Defense officials said.
Officials familiar with the military surveillance efforts say they will be able to do much more than watch the crowd. They can sense radiation associated with a dirty bomb and in some cases detect a conventional explosive device.
In addition to response, officials have worked on prevention. Authorities have been scouring the Internet and other extremist gathering places to look for signs of trouble. So far, they say, they aren't seeing any.
Domestically, white supremacists have discussed Obama in threatening terms since early in his candidacy. Threats increased after Obama won the Democratic primary and again after he won the election.
Obama has been a lightning rod not only because of his race, but due to what extremists believe to be his ties to Islam, perceived sympathy for Israel and even support for gun control measures.
But in recent weeks, the threatening "chatter" has died down, according to law enforcement officials and the Southern Poverty Law Center, which is active in tracking U.S.-based extremist activity.
Yet current and former security specialists say that such screening procedures usually can't catch the kind of zealots Obama might attract.