At least four people died and several reportedly remain trapped Sunday when part of a highway tunnel collapsed in eastern Japan, the most serious accident of its kind in the country in more than 15 years, media reports said.

A portion of the ceiling of the Sasago tunnel, which is 4.7 km (2.9 mi.) long and is located about 80 km (50 mi.) from Tokyo, collapsed early Sunday morning for still unknown reasons, trapping three vehicles, one of them a van.

One of the vehicles caught fire and burned unchecked, and authorities later found three bodies inside the charred wreckage, police said.

The fourth fatality was the driver of a truck who had managed to make a distress call after the accident from his mobile phone, but he stopped responding to incoming cell phone calls shortly thereafter.

Some 150 cement panels from the roof of the tunnel gave way and fell onto the roadway along a stretch about 100 meters (about 110 yards) long, said the Central Nippon Expressway company, which operates the highway.

It is thought that there are still at least three people trapped - or dead - under the fallen cement panels, which some witnesses said hit at least three vehicles.

The ceiling collapse blocked the tunnel and also caused another small vehicle to burst into flames, although that fire was quickly put out, but the smoke and the chaos that followed the accident spurred many drivers to abandon their vehicles and move out of the tunnel on foot. EFE