Stumped for years by blatant suspects, police are using forensic analysis to hunt down gropers on crowded trains by matching tiny fabric fibers from the perpetrator's palm with the victim's clothes.

The technique used by Osaka Prefectural Police helped nab 16 suspects in the first 10 months of this year, up from four cases proved the whole of last year, said Hirotsugu Sanuki of the force's science research institute.

Osaka police began applying the method in 2003, Sanuki said.

Groping has long been a problem on crowded subways and other commuter trains in big cities like Osaka and Tokyo. Passengers are often tightly pressed against each other, an inviting environment for potential offenders.

Under the technique, a special film is pressed on the suspect's palm to collect microfibers about a few hundredths of a millimeter. The samples taken are compared with fiber samples from the suspect's clothes on a powerful microscope, Sanuki said.

A similar investigation method is reportedly being used by Tokyo police in about 150 molestation cases a year.

A record 2,201 cases of groping on Tokyo commuter trains were reported to police last year, the latest year in which police statistics were available. More than half of them were committed during the jam-packed morning rush hour.

According to Osaka police, 416 gropers were arrested last year, up 8 percent from the previous year.