Two koalas that spent their summer visiting Boston's Franklin Park Zoo on loan from the San Diego Zoo, are having a hard time getting through security to fly home.

The sensitive, valuable marsupials cannot be flown in the cargo area and instead fly with the passengers, zookeeper Brian Rutledge said.

Since the Sept 11. attacks when hijackers crashed airplanes into the World Trade Center and the Pentagon, airport security has been tightened at U.S. airports.

Two of the four planes -- American Airlines Flight 11 and United Airlines Flight 175 -- were hijacked from Boston's Logan International Airport.

American Airlines flew the pair for free from San Diego earlier this year and was to fly them home.

But American, like other airlines, is now running on a reduced schedule and cannot promise that the wide-body jets necessary to fit the crates will be available for a San Diego run when the animals are slated for return, a spokeswoman told the newspaper.

Heightened security was also complicating the problem, the spokeswoman said.

The 8-pound (4-kg) koalas are not ferocious, but their crates cannot be searched with them inside. The animals cannot be taken out of the crates at the airport because of the stress it would place on them and the risk they could get loose.

``Searching their pockets is out of the question,'' Rutledge told the newspaper.

Trains and buses take too long and also would put stress on the animals that are native to Australia, zoo officials said. Their shelter at the Boston zoo was built for summer in New England, not winter. The zoo said it would find a way to keep the koalas warm when it turned colder.

The San Diego Zoo told the newspaper it would wait patiently for 20-month-old Kiley and 2-year-old Pendicup.

(Agencies)