Terrorists Standing by on American Soil?

Terrorists Standing by on American Soil?

Warnings about the threat of terrorism here in the homeland keep coming in.

The latest warning comes from the Director of National Intelligence Mike McConnell.

"The most serious threat is that the plotters that are being observed will be successful in penetrating our defenses and conducting attacks that will result in mass casualties."

McConnell says he believes al-Qaeda, in some cases, already has people positioned in the U.S., where small numbers of operatives are raising funds for attacks on American soil.

"Their intent is to effect an attack with mass casualties, secondary attempt would be political or infrastructure targets," he told NBC's Meet the Press.

Last week, McConnell specifically mentioned targets such as Chicago's Sears Tower or other prominent buildings in Seattle, Los Angeles, or Dallas.

Another major worry: leading scientists have told ABC News that the U.S. is still vulnerable to attempts to smuggle nuclear material into the country.

The warnings come on the heels of last week's National Intelligence Estimate in which the nation's 16 spy agencies agreed al Qaeda has strengthened its ability to attack the United States - and has regrouped in the mountainous frontier between Pakistan and Afghanistan.

It is here where intelligence analysts believe Osama bin Laden, himself, is hiding and an area in which the Bush administration has not ruled out military strikes.

"Just because we don't speak about things publicly, doesn't mean we're not doing many of the things you're talking about," Homeland Security Advisor Frances Townsend said.

But Pakistan refutes that bin Laden is in the country in what is becoming a diplomatic row between the U.S. and a key ally in the war on terror.

"We can't allow another safe haven to exist. But if we do things to prevent that safe haven from being created that undermine the government, we create a bigger safe haven with an unstable Pakistan," Sen. Evan Bayh, D-Ind., said.

Administration officials acknowledge growing concern about unrest in Pakistan and say it could have a quote severe impact on the U.S.- led global war on terror.