About a dozen uniformed police were shown on television Tuesday entering the company's headquarters in the central city of Nagoya, toting cardboard and plastic boxes.
"Yes they are searching our offices here. We will be fully cooperating with them," said Osamu Funahashi, another company official.
The slabs, each weighing 1.4 metric tons (1.54 short tons), fell over a stretch of about 110 meters (120 yards). They had been suspended from the arched roof of the tunnel.
The operator was exploring the possibility that bolts holding a metal piece suspending the panels above the road had weakened with age. The panels, measuring about 5 meters (16 feet) by 1.2 meters (4 feet), and 8 centimeters (3 inches) thick, were installed when the 4.7-kilometer-long (3-mile-long) tunnel was built.
Recovery work in the tunnel about 80 kilometers (50 miles) west of Tokyo halted Monday to allow reinforcement of the roof to prevent more collapses, said Jun Goto, an official at the Fire and Disaster Management Agency.
By Tuesday, crews were removing the concrete slabs from the tunnel, said Goto, who added that authorities do not expect to find any more victims inside.
Spending on public works once was the lifeblood of the conservative Liberal Democratic Party, which has ruled Japan for most of the decades since in World War II, though it was ousted by the Democratic Party of Japan in 2009.
The government created huge oceanside reclamation projects, bullet train lines and other vital infrastructure, as well as notorious "bridges to nowhere."
Political reforms beginning in the early 2000s focused on cutting spending on public works, but they failed to differentiate between projects that contributed to efficiency and competitiveness and those that did not, said Masahiro Matsumura, a politics professor at St. Andrews University in Osaka, Japan.
"Basically, we didn't spend enough on renovating our decaying water pipes, bridges and tunnels. We didn't spend enough on public infrastructure," Matsumura said.