BEIJING - An aircraft maintenance company in South China is conducting an internal investigation into why 30 screws were recently missing from the wing of an Air France jet.

The four-engine Airbus 340, which can carry 360 passengers, was recently reported to have been grounded in Boston in mid-November because a third of its screws were found to be missing from a large protective panel on one of its wings.

Before then, the plane had undergone maintenance in Xiamen in Fujian province and stopped at Air France's Paris hub for a few days.

Taikoo Aircraft Engineering (TAECO) in Xiamen said on Tuesday that the plane had stayed at the company's base for routine maintenance between Oct 4 and Nov 10.

The company is now carrying out an internal investigation into why the screws were missing, said TAECO, which is a subsidiary of Hong Kong Aircraft Engineering Co Ltd.

"We will follow our standard procedures and talk directly to Air France about the investigation results and determine our subsequent measures as fast as we can," the TAECO statement said.

The screws were to help secure a panel that is intended to reduce the plane's drag during flight. TAECO quoted Air France as saying that the absence of the screws did not endanger the flight.

Industry insiders said it is very rare for screws to be found to be missing from a plane that has undergone routine maintenance.

Lu Xiang, a lecturer at Civil Aviation University of China specializing in airplane structure, said planes that have had maintenance performed on them usually are also checked to make sure that maintenance was done properly.

"It is absolutely forbidden to let planes out with a blunder like this," he said.

Various foreign media outlets have reported on the case, many of them laying emphasis on the fact that the screws were found to be missing after the plane had undergone maintenance in China.

Chinese netizens reacted strongly to the news. Some said they would not be surprised if maintenance workers were to blame for the screws' absence, especially in a country "where sometimes a surgeon leaves gauze in a patient's belly".

Others, though, said the screws might have become unattached some time after the maintenance had been performed.

"Why did Air France fail to find out the missing screws when the jet left Paris for Boston? Please be objective," an unidentified netizen wrote on micro blog on Tuesday.

Lu said blame should be assigned in accordance with the investigation results.

Founded in 1993, TAECO is one of the first foreign companies in Xiamen. It is now one of the largest plane maintenance companies on the mainland.