TOKYO — Nine people were found dead early Monday after their cars were crushed by slabs of concrete that fell from the ceiling of a tunnel in eastern Japan, severing a major highway leading to Tokyo.

The police said they were investigating the cause of the collapse on Sunday at the Sasago Tunnel — a three-mile passage near the city of Otsuki and about 50 miles west of Tokyo — and for evidence of negligence by the company that operates the highway.

News reports said investigators believed that supports in the ceiling of the 35-year-old tunnel might have grown brittle, allowing hundreds of the slabs to fall onto passing vehicles. Each slab weighed 1.2 tons, officials said.

Image The highway connects Tokyo and western Japan.

The concrete crushed three vehicles, including a van carrying six people that caught fire, filling the tunnel with thick, black smoke. Only one of the van’s occupants, identified as a 28-year-old woman who worked at a bank in suburban Tokyo, was able to escape, the police said.