US investigators and civil aviation officials in Africa said the disappearance was probably criminal in nature, but they have not ruled out a link to terrorism with the jetliner that was converted into a fuel tanker.
"There is no particular information suggesting that the disappearance of the aircraft is linked to terrorists or terrorism, but it's still something that obviously we would like to get to the bottom of," said state department spokesperson Philip T Reeker.
US officials speaking on condition of anonymity said a variety of investigative and intelligence-gathering methods are being used in the search. But they declined to provide details.
But experts said even in the age of satellites and other high-tech search methods, all that is needed is a new coat of paint and a stolen registration number to make tracking the plane near impossible.
"Let's assume (the pilot) did arrive in some place like Nigeria ... a couple of thousand of dollars changed hands and the aircraft is put in a hanger. The chances it is seen before satellites get a chance are zip," said Chris Yates, the Jane's Aviation and Security editor in a phone interview from London.
"It's happened before in African aviation," he said.
The plane, with tail number N844AA, left Luanda airport on May 25. The transponder was turned off so the plane was not tracked by air traffic control, US officials said.
Over Africa's vast and often desolate terrain, tracking by radar is problematic at best.
"We are mystified to as to where (the plane) might have gone," said Richard Cornwell, a senior researcher at the Pretoria-based Institute for Security Studies.
But Cornwell said if someone wanted to hide a plane, Africa would be the place to do it. Radar in the African skies is often "nonexistent" he said.
"Pilots talk about flying the gauntlet between South Africa and North Africa. There is no (air) control, even on commercial levels."
In the shadow of the September 11, fears of airborne attacks remain high.
Last month US authorities said they uncovered an al-Qaeda plot to crash an explosives-laden small aircraft into the American consulate in Karachi, Pakistan.
The US Homeland Security department issued an advisory stating al-Qaeda had a "fixation" on using aircraft in attacks.
The plane's conversion to a tanker has added to the worries.
"If you fill that up with however many gallons of jet air fuel and stick a couple of suicide pilots on, it doesn't take Einstein to figure out you could fly into an American or British embassy or another target they want to strike against. It could be a huge bomb," said Yates.
US analysts, however, believe the plane was taken by some criminal venture or in some business or insurance dispute. It is also possible it crashed. According to media reports, it's last radio contact asked for landing permission in the Seychelles, but it never arrived.
US Federal Aviation Association records show the aircraft - formerly owned by American Airlines - was most recently owned by Aerospace Sales and Leasing Company Inc of Miami, Florida.
The company's listed phone number in Miami has been disconnected.
The family of 51-year-old Ben Padilla who presented himself to Angolan Civil Aviation authorities as the representative of the plane's owner was seen boarding the plane along with another man shortly before the plane took off May 25, Angolan aviation officials said.
According to his family in Florida, he was hired to repossess the plane after Air Angola failed to make lease payments.
His sister, Benita Padilla-Kirkland, told the South Florida Sun-Sentinel newspaper she feared the plane had crashed or that he is being held against his will.
Helder Preza, director of Angola's civil aviation authority, told The Associated Press that the plane had been grounded for about a year because it lacked proper documentation verifying its legal conversion to a tanker.
He said Padilla approached authorities a month before the plane disappeared, saying the owner wanted to take the plane out of Angola.
"We said no problem," Preza said, as long as he could pay $50 000 in duties for the aircraft for the year it sat in Angola and that he could provide proof Air Angola approved.
In the meantime, Padilla asked airport authorities in Luanda to do maintenance on the plane.
Preza said it was it was during the maintenance process that Padilla or another man started the engines and then flew the plane away.
Air Angola, an airline reportedly owned by army officials, has been in financial distress since the signing of peace accords last year that ended 25 years of civil war. Peace brought an end to lucrative contracts for wartime transportation.
The airline also has some links to shadowy figures in the world of arms running, Yates said.
In the past the airline has bought planes from companies partially owned by Victor Bout, an alleged Russian arms dealer accused by Belgian authorities of shipping weapons to the al-Qaida terrorist network.
Bout, a former Soviet air force officer turned international businessman, denies that he had been involved in smuggling weapons to al-Qaeda although he ran cargo flights to Afghanistan during the rule of the Taliban. According to UN investigators, his air cargo empire, built on old Soviet aircraft, violated United Nations arms embargoes and delivered weapons to rebels in Sierra Leone, Congo, Angola and Rwanda.
"There is no strong evidence that there is a connection," Yates said. "But it seems with the Air Angola connection and knowledge of illegal activities across Africa, it's entirely possible there is some sort of tie."
Phone calls to the Air Angola office in Luanda were not answered. - Sapa-AP