Public prosecutors on Friday decided not to indict any of the 10 people accused of professional negligence over the collapse of an expressway tunnel that killed nine people and injured three others in Yamanashi Prefecture in December 2012.

The Kofu District Public Prosecutor’s Office found it difficult to prove the guilt of the accused, including Takekazu Kaneko, former president of Central Nippon Expressway Co., and Nexco Central, operator of the Sasago Tunnel on the Chuo Expressway, as well as maintenance supervisors at the firm and a Nexco Central subsidiary.

The prosecutor’s office investigated whether there was negligence in the tunnel’s design, construction and maintenance. The office also heard from experts.

It concluded that it was difficult for the accused to predict the tunnel would collapse, sources in the office said.

On Dec. 2, 2012, portions of the tunnel’s concrete ceiling collapsed over the Chuo Expressway’s Tokyo-bound lanes, crushing three vehicles with heavy slabs. Nine people were killed and three injured.

The Sasago Tunnel links the cities of Otsuki and Koshu.

In November, Yamanashi police sent the prosecutor’s office documents on Kaneko and three others who were suspected of not conducting sufficient inspections and safety checks on bolts used to hold the metal rods of the ceiling slabs. Files were also handed over on four other suspects who allegedly skipped part of the examination and overlooked the degradation of the bolts.

Separately, criminal complaints were filed with the prosecutor’s office against two other people.

“We have hoped that a trial on the case would begin and the truth about the accident would be uncovered, so today’s decision by the prosecutors is regrettable,” relatives of the victims said in a statement released Friday.

The fact that no one will be held responsible for the fatal accident showed the limits of Japan’s judicial system, they said, adding that a similar incident could happen unless the truth about the Sasago Tunnel collapse is discovered.