(CNN) The military is investigating why an aging cargo plane, perhaps making its final military flight, nosedived Wednesday into a Georgia highway, killing all nine people on board.
"Nine crew members died in the accident, but until their families and relatives are notified, we cannot give their names," said Brig. Gen. Isabelo Rivera, commanding officer for the Puerto Rico National Guard.
All were from Puerto Rico, officials said. The military is still in the process of notifying their next of kin, Col. Pete Boone, vice commander of the Savannah, Georgia-based 165th Airlift Wing, said Thursday morning.
Puerto Rico Gov. Ricardo Roselló declared nine days of mourning for the crew, during which flags in the territory will fly at half-staff, according to a statement from his office.
The Puerto Rico Air National Guard WC-130 was flying from Savannah to Tucson, Arizona, when it crashed. Though an official familiar with the aircraft told CNN the plane was at least 50 years old, Boone told reporters Thursday the plane was manufactured in the late 1970s.
Boone also said he was unsure if the plane was flying to Arizona to be decommissioned, despite reports to that effect.
The wreck left a debris field of 360,000 square feet -- about the area of six football fields -- Chatham County officials said. Georgia Highway 21 will remain closed indefinitely as investigators examine the crash site and debris field, they said.
The investigation is being carried out by the National Guard Bureau and the Air Force, Rivera said. A team from Charleston Air Force Base in South Carolina is conducting the investigation, while a team from Dover Air Force Base in Delaware was sent to recover the airmen's remains, Boone said.
The crash occurred moments after takeoff near Hilton Head Airport in Savannah, Rivera said.
There were no injuries on the ground.
"We don't know the cause of the crash," said Maj. Paul Dahlen, spokesman for the Puerto Rico National Guard.
The plane, from the 156th Airlift Wing in Puerto Rico, had been in Savannah for "a number of days" undergoing routine maintenance before heading to the Aerospace Maintenance and Regeneration Group in Arizona, he said. AMARG, also known as "The Boneyard," is an Arizona facility where the military keeps thousands of aircraft, as well as spacecraft.
Boone said he couldn't specify what type of maintenance the aircraft underwent.
Video from a business near the crash showed the final moments of the hulking plane, a version normally outfitted to do weather reconnaissance.
The aircraft, with four turboprop engines on its overhead wing, banked left as it rapidly descended. The plane then plummeted behind trees. Seconds later, a fireball and thick black smoke erupted from the tree line.
Truck driver Roger Best, who works for a transportation company in Garden City, about 4 miles south of the crash, saw the plane overhead before it crashed. It was so low when it passed over that it rumbled the ground beneath his rig, he said.
His company handles hazardous materials, he said, expressing gratitude that the aircraft didn't crash in his immediate vicinity.
"If he did, this area is a giant bomb, and this whole (5-mile) radius would have blown up," he told CNN over Facebook, calling the pilot a hero for avoiding the area. "He barely made it over the tree line, (and it) looked as if he tried to turn and nosedive straight into the ground right in front of me."
"The ground shook like a bomb was going off," Denver Goodwin, who works down the street from the crash, said. "All the people in the building started panicking. It was absolutely horrible."
A tweet from the Savannah Professional Firefighters Association showed the tail of the plane protruding from thick smoke and flames.
The WC-130 that crashed was used during Hurricane Maria for relief efforts, the Puerto Rico National Guard said. The plane also was used during Hurricane Irma to rescue US citizens from the British Virgin Islands.
WC-130 is a variant of the C-130. Produced continuously since 1954, the C-130 is a reliable and versatile aircraft with several iterations. It can be outfitted for transport, reconnaissance, search and rescue, research, refueling, patrol or as a gunship.