A government expert has conceded the picture on wrongly jailed teenager Lin Qiaoying's passport had an irregular feature - but said it was not necessarily caused by photograph substitution. Leung Yee-mei, a document examiner for the government laboratory's forensic science division, was being cross-examined by defence lawyers in the District Court yesterday. On Thursday she had asserted the mainland passport of Ms Lin, 18, showed no irregularity or sign of forgery. Edward Mumford SC, counsel for part-time interpreter Wong Kin-ang, 45, said a document expert called for the defence had said there was a cut mark above the edge of the passport photograph. He alleged it was a feature of photograph substitution. Mr Mumford suggested the picture had been swapped and the passport used by another person to sneak into the United States. The passport was then returned to Ms Lin, who replaced her photograph, he said. Asked if the cut mark was made by photograph substitution, Ms Leung said: 'I agree it's one of the scenarios that may leave a cut mark.' But she said other irregular features that usually indicated substitution were missing. She also agreed that air bubbles on the laminated part of the passport were unusual. The court has heard Ms Lin was threatened into making a false confession that she was, in fact, another girl and had bought a forged passport in Fuzhou. She was sentenced to four months' jail but had her conviction quashed after her passport was found to be genuine. Wong and immigration assistant Wong Chui-kam, 28, deny perverting the course of justice. Their trial continues before Judge Barnabas Fung Wah.