A bus explosion that killed 23 nursing home patients fleeing Hurricane Rita exposed flaws in government oversight of motor carrier companies, safety investigators said Wednesday.

The rear wheel of the bus caught fire in the early morning of Sept. 23, 2005, on a freeway near Dallas. Within minutes the vehicle was engulfed in flames and smoke, investigators said during a National Transportation Safety Board hearing on the probable cause of the accident.

Though the bus company, Global Limo Inc., didn't conduct required preventive maintenance or keep inspection records, a state safety inspection three months before the accident gave it a satisfactory rating, NTSB member Kitty Higgins said.

Higgins described the bus company as "a poster child for what not to do."

"If we can't get the worst of the worst out of business, what's the value of an oversight program?" said NTSB member Deborah Hersman.

Higgins said the heart of the accident was a lack of maintenance. Investigators said they'd found the wheel hub had no lubricant, causing the wheel bearings to fail which in turn created friction and heat.

Investigators said daily inspections, recommended by the bus manufacturer, would have revealed the lack of lubricant. The driver didn't inspect the bus, investigators said.

Saying the bus company, Global Limo Inc., was a hazard to the public, federal regulators shut it down weeks after the accident. Global Limo's owner James Maples was convicted of maintenance and inspection charges in October.

Victims and relatives of victims reached an $11 million settlement in May with Global Limo and BusBank, the travel broker that hired it.

The fire probably caused the patients' oxygen canisters to explode, making it even harder to evacuate them from the bus, investigators said.

Since the accident, the Department of Transportation has issued new guidelines for carrying medical oxygen, recommending that tanks be secured in an upright position and limited to one canister per patient in the passenger compartment.