(Recasts to add retirement flight element; adds details on victims)
By Phil Stewart
May 3 (Reuters) - All nine people on board were killed when a Puerto Rico Air National Guard cargo plane, on what was to be its last flight before retirement, crashed on Wednesday near Savannah, Georgia, scattering fiery debris over a highway and railroad tracks, authorities said on Thursday.
Officials said the plane, a Hercules WC-130, was believed to be around 50 years old but was in good mechanical condition, and was making what had been scheduled to be its last flight.
They said it was too soon to speculate about the cause of the crash. The WC-130 is a transport workhorse of the U.S. military.
The plane went down shortly after take-off from the Savannah/Hilton Head International Airport, about 175 miles (280 km) southeast of Atlanta, officials said.
All nine crew members died, Brigadier General Isabelo Rivera, assistant general of the Puerto Rico Air National Guard, said in a statement.
Officials identified the nine victims, all members of the Puerto Rico National Guard, who had served in the military from three to 31 years. Five were crew members and four were passengers who were military maintenance and operations personnel. Eight had children.
"Taking care of our fallen Airmen's families and loved ones is our top priority," Rivera said in a statement.
He said the National Guard and U.S. Air Force had launched an investigation to determine the cause of the crash.
The plane, which was on a training mission, was headed to the Aerospace Regeneration and Maintenance Group in Tucson, Arizona, where it was to be retired after Wednesday's flight, Major Paul Dahlen, spokesman for the Puerto Rico National Guard, told Pentagon reporters.
"It was basically its last flight," Dahlen said. "Although it was an older aircraft, it was in good mechanical condition."
The four-engine plane sent up a towering cloud of black smoke, with a tail wing coming to rest on a highway median, television images showed. A witness, Michael Garrett, told WSAV-TV on Wednesday the plane was upside down before it crashed.
"That plane really flipped over on its back, slowly, like it was in slow motion," he said.
Gena Bilbo, spokeswoman for the Effingham County Sheriff's Department, told reporters it was "an absolute miracle" that no vehicles were hit in the busy intersection at the crash site.
The crash was at least the fifth fatal accident involving a U.S. military aircraft since early April.
In July last year 16 service members were killed in a KC-130 crash in Mississippi. (Reporting by Phil Stewart in Washington Additional reporting by Ian Simpson in Washington, Bernie Woodall in Fort Lauderdale, Fla, and Gina Cherelus in New York Editing by Frances Kerry and Dan Grebler)