Johannesburg, South Africa ? In a brazen act, two men climbed aboard an idle Boeing 727 cargo jet in Angola last month and flew off into the African sky without a trace.

The disappearance touched off searches across the continent and, in the post-Sept. 11 era, prompted worries about why the plane was taken.

U.S. investigators and civil aviation officials in Africa said the plane most likely was taken for a criminal endeavor such as drug or weapons smuggling, but they have not ruled out the possibility it was stolen for use in a terrorist attack.

“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 a State Department spokesman, Philip T. Reeker.

Experts said that even in the age of satellites and other high-tech search methods, just a new coat of paint and a stolen registration number would make tracking the plane nearly impossible.

“Let’s assume (the pilot) did arrive in some place like Nigeria … a couple of thousand dollars changed hands and the aircraft is put in a hanger. The chances it is seen before satellites get a chance are zip,” Chris Yates, editor of Jane’s Aviation and Security, said in a phone interview from London.

The plane, with tail number N844AA, left Luanda airport May 25. The transponder was turned off, so the plane’s position could not be monitored by air traffic control.

Last month U.S. authorities said they had uncovered an al-Qaida plot to crash an explosives-laden small aircraft into the American consulate in Karachi, Pakistan. The U.S. Homeland Security Department issued an advisory saying al-Qaida had a “fixation” on using aircraft in attacks.

The fact that the missing 727 had been converted into a fuel tanker has added to the worries.