TOKYO-- Train operator West Japan Railway Co. said Tuesday that a 16 centimeter crack found in a steel frame of a carriage of a shinkansen bullet train was "extremely serious."

JR West Vice President Norihiko Yoshie also told a press briefing on the matter that the incident had caused public mistrust in the safety of Japan's shinkansen trains.

"It was an extremely serious incident that betrayed the trust in the safety of shinkansen. I deeply apologize," Yoshie was quoted as saying.

The crack was originally detected on Dec 11 after the crew from the Nozomi No 34, bound for Tokyo from Hakata, Fukuoka Prefecture, noticed an acrid, burning smell and heard that one of the motors sounded unusual.

The bullet train, according to local media accounts, finally stopped at Nagoya Station, after operating for another 3 hours.

If the train had continued to run, the crack could have caused it to derail, transport officials said at the time.

On inspection, JR West maintenance crews found the 16 centimeter crack on the bottom of the steel frame of one of the carriages. They also found the frame was heavily damaged on both sides.

The train was moved from Nagoya Station to a train yard in Hakata, for a full inspection which lasted a week.

"We recognize it as a 'serious incident' as it was an extraordinary case. It could have led to an accident" of the nation's iconic, high-speed bullet trains, said an official from the Japan Transport Safety Board.