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.
