(CBS) –A transportation watchdog group tells CBS 2 it believes a design problem likely led to a fire on a Megabus over the weekend.

The group says it is focusing on the build-up of intense heat under the rear of many of these buses. CBS 2’s Mike Parker explains in this Original Report.

Amazingly all 40 or so passengers escaped the fast-moving flames thanks to the driver who helped them evacuate in time. Calls to 911 came pouring in.

An image of the wheel well suggests the fire could have started there. Parker talked with a spokesman for an industry watchdog group.

“What can happen is if a wheel hub is either low on oil, it can result in an elevated temperature within the hub which can ignite any of the surrounding combustible material,” said Collin Mooney of the Commercial Vehicle Safety Alliance. “Once a tire starts to burn, it’ll burn until there’s no material left.”

So is it safe to use long-distance buses? It depends on who you talk to.

Yes says the American Bus Association citing safety statistics from the federal government. But consumer advocate Clarence Ditlow of the Center for Auto Safety in Washington says no. Megabus in particular, he says, makes more money by overloading its buses. They can overheat and become infernos.

A Megabus spokesman says the company is cooperating fully with the investigation.