A senior US administration official tells Pentagon correspondent Barbara Star that it is the 'origin' of the intelligence information about a possible 9-11 attack that is causing the concern that has led the warnings from the Obama Administration.
"It gives us pause" he says.
"It all gets to the assessment of where the information is coming from" the source said.
The focus right now is finding corroboration that either this is real, or they can dismiss it, the source said.
The 'working assumption' is that operatives have entered the US. "The working assumption is if the plot is for real and there are individuals plotting then some have already arrived." But he stressed they cannot confirm that has happened.
The US also believes the operatives came out of the Pakistan-Afghanistan border region, that they are part of Al Qaeda 'central", but he said there is also a possibility Pakistan-based groups like LET or TTP groups are also involved.
Asked by CNN business reporter Alison Kosik if Pakistan was involved in uncovering the threat or following up on it, Secretary of State Hillary Clinton said U.S. government is taking the threat seriously.
"We are, obviously, working with everybody to try to find as much information as possible," Clinton said in an interview after she helped ring the opening bell at the New York Stock Exchange.