Sunday's expressway tunnel collapse in Yamanashi Prefecture that killed at least nine people may have been caused by aging ceiling bolts that failed, Central Nippon Expressway Co. said Monday.

Facing reporters Monday in Nagoya, where the highway operator is headquartered, Ryoichi Yoshikawa, CNEC executive in charge of maintenance, said failed bolts were found at the site where tons of concrete ceiling panels fell onto vehicles traveling through the Sasago Tunnel.

"Superannuated (bolts) may be" the cause of the tunnel collapse, Yoshikawa said, suggesting the bolts were never replaced. "There is no record that shows repair work was carried out in the past."