The United States says it is working with African governments to try to find a stolen passenger jet that it fears may end up being used by terrorists.
The plane has not been seen for four weeks
White House spokesman Ari Fleischer admitted the US had few leads over who was behind the theft of the plane, from an airport in Angola last month, but was keen to gather any information available.
"We don't have any reliable assessments about what this portends, what it could be, who may be behind it. But it is an issue that is being worked on," he said.
His comments follow a report in the Washington Post newspaper on Wednesday that the Central Intelligence Agency and the State Department had joined a continent wide hunt for the aircraft.
The paper quoted American officials as saying that - in the worst case scenario - the plane could be used in an 11 September-style attack.
There is also concern about a missing US pilot who had been sent to Angola by a plane-leasing company in Florida.
The US authorities are said to believe he was at the controls when it left Luanda.
The family of the man, Benjamin Padilla, have told ABC News they thought he may have been kidnapped.
They said he only went to Angola to check if the plane was airworthy.
Angolan state radio said shortly after its disappearance that the jet had been chartered by an Angolan airline but was grounded after a number of irregularities.
The Boeing 727 took off from the Angolan capital on 25 May after being at the airport for 14 months.
When it started taxiing down the runway, the radio control tower tried to make contact with the pilot, but there was no response and nothing has been seen of the plane since.
Since then, US spy satellites have taken pictures of remote airstrips throughout Africa and US diplomats have been seeking the aircraft.
The South African aviation authorities say they have a sophisticated monitoring system and would know if the missing plane entered their airspace.
Originally owned by American Airlines, when the 727 was the world's best selling passenger jet, the plane was subsequently leased and subleased by a number of people and companies.
A company called Miami-based Aerospace Sales and Leasing Co. is reported to be its current owner.
African airports are littered with old planes that have proved too costly to maintain and keep in the air.