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By Alexander Smith

South Africa's national airline said Friday it suspects a crime syndicate may be operating on one of its routes after passengers were hit with a spate of high-altitude thefts.

South African Airways (SAA) has blacklisted one passenger and said it was looking at introducing additional security measures after three thefts in as many weeks on its Johannesburg to Hong Kong route, spokesman Tlali Tlali told NBC News.

South African Airways planes sit on the tarmac at O.R. Tambo International airport in Johannesburg in February 2015.Bloomberg via Getty Images

The incidents happened on either side of New Year and only one suspect has ever been detained.

A Hong Kong resident was allegedly found in possession of cash and jewelry belonging to another flyer on a plane into Johannesburg on Christmas Eve, according to Tlali.

He was restrained on the flight but later released on landing because the alleged victim, a vacationer, did not want to press charges, the spokesman said. This was because doing so would have involved temporarily handing over the cash to police, which they deemed too inconvenient.

Although police took no action, the man was blacklisted by the airline.

"The incidents occurred within a short space of time on one route to give rise to suspicion that a syndicate could be involved," Tlali said in an emailed statement.

"We are extremely concerned that we have experienced such unfortunate events that leave a bad after taste on our passengers’ flight experience," he added. "We sincerely apologize for these and are exploring a number of practical interventions to eliminate their re-occurrence."