Investigators Saturday picked over debris from the explosion of a propane truck that blew up and burned after hitting a retaining wall on a downtown interstate, killing eight people.

Seven of the dead were killed as a result of the blast and fire; an eighth died in the resulting traffic jam, authorities said.Experts from the National Transportation Safety Board were called in, and "this morning we met with the various parties or people who have involvement in this accident and organized the investigation," said spokesman Michael Benson.

The accident involved "interstate trucking and hazardous materials and that's why we're concerned about that," he said.

Investigators will examine the record of the truck and driver, the trucking company, the layout of the interstate and the response of emergency personnel, he said.

"We'll be looking at probable cause and any safety issues that may have relevance anywhere else in the country," Benson said.

The 100,000-gallon tanker truck skidded on a 25 mph exit ramp of Interstate 240 in the Midtown section of Memphis, struck a retaining wall and exploded, said fire department Capt. Benny McDow.

The tank shot 125 yards and crashed into a duplex apartment, crushing the building and killing a 10-year-old girl inside. A mammoth fireball set several other houses and nearby cars on fire.

Benson said six NTSB investigators will head teams of local and state authorities.

"The field phase of the investigation may take up to a week. The entire investigation will take up a minimum of nine months," Benson said. "We go into it very thoroughly."

The dead included Randall Benson, 29, West Memphis, Ark., the truck's driver; Harrison G. Lee, 34, Brighton, Tenn., a motorist; Robert Wardlow, 40, Memphis, a motorist; Tina Wiles, 30, Vicksburg, Miss., a motorist; Warner Wiles, 70, Vicksburg, Miss., Mrs. Wiles' father-in-law; and Shelanda Towles, 10, killed in her house.

Iva J. Rubesheim, 87, who lived in the other half of the duplex from Shelanda, died later in the Regional Medical Center.