TOKYO -- At least seven people were feared dead after a highway tunnel collapsed on Sunday in eastern Japan, trapping them in their vehicles and starting a fire that filled the passage with thick, black smoke.

Three vehicles appeared to have been crushed under concrete that fell from the ceiling of the three-mile Sasago Tunnel near the city of Otsuki in Yamanashi Prefecture, about 50 miles west of Tokyo, the national disaster management agency said. Officials from the agency and the police said it remained unclear why a section of the tunnel -- 150 to 200 feet of eight-inch-thick concrete that weighed about 180 tons -- suddenly fell.

A vehicle carrying six people caught fire, whose heavy smoke initially prevented firefighters from entering the tunnel. But even after putting out the blaze, rescuers had to temporarily suspend efforts to reach the vehicles inside because of the danger of a further collapse, officials said.

Rescue efforts resumed later in the day, though progress was slow.

A 28-year-old woman managed to escape from the vehicle that caught fire, officials said. She told firefighters that five other people remained inside. It was unclear how many people were in the other trapped vehicles besides the drivers.

One of the other vehicles appeared to be a truck belonging to a food wholesaler, officials said. The truck's driver called his company right after the accident to ask for help, they said, but officials were later unable to reach him on his cellphone.

The operator of the highway, the Central Nippon Expressway, held a news conference to apologize for the accident. The police said they had opened an investigation into the cause of the collapse and about whether professional negligence by the operator was a factor.

The accident closed a section of the Chuo Expressway, which connects Tokyo to western Japan. Such long tunnels -- usually lined with smooth, white concrete -- are common on highways in the mountainous island nation.

First Published: December 2, 2012, 11:00 p.m.