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.