Two vehicles caught fire in the accident Sunday morning, and heavy smoke initially hindered rescue efforts. The location of the accident about 1.7 kilometers (a mile) inside the 4.7-kilometer (3-mile) long Sasago Tunnel was also making the work difficult.
The nine dead were traveling in three vehicles in the tunnel about 80 kilometers (50 miles) west of Tokyo on a highway that links the capital to central Japan. The tunnel opened in 1977 and is one of many in the mountainous country.
The search was suspended Monday morning while the highway operator does work to support the remaining slabs in the ceiling, said Jun Goto, an official at the Fire and Disaster Management Agency. It's expected to resume by afternoon.
Goto said it's not clear if there are other survivors.
Police and the highway operator Central Japan Expressway Co. were investigating why the concrete slabs collapsed. An inspection of the tunnel's roof in September found nothing amiss, according to Satoshi Noguchi, a company official.
An estimated 270 concrete slabs, each weighing 1.4 metric tons (1.54 short tons), suspended from the arched roof of the tunnel fell over a stretch of about 110 meters (120 yards), Noguchi said.
The operator was exploring the possibility that bolts holding a metal piece suspending the panels above the road had become aged, he said. The panels, measuring about 5 meters (16 feet) by 1.2 meters (4 feet), and 8 centimeters (3 inches) thick, were installed when the tunnel was constructed in 1977.
Company President and CEO Takekazu Kaneko said that the company was inspecting other tunnels of similar structure, including a parallel tunnel for traffic going in the opposite direction. Both sections of the highway were shut down indefinitely.
Two people suffered injuries in the collapse.