The plane that crashed near Savannah and killed all nine on board was making its last flight and was about to be retired, an Air Force official told ABC News Thursday.The WC-130 cargo plane was headed to the so-called "boneyard" at Davis Monthan Air Force Base near Tucson, Arizona, but crashed shortly after taking off from the Savannah/Hilton Head International Airport.The "boneyard" is the holding facility at the base for 4,400 old aircraft that are stored for spare parts or future use.The revelation that the fatal flight was already slated to be the plane's last comes after Col. Pete Boone, director of Air Operations and vice commander of the Savannah-based 165th Airlift Wing of the Georgia Air National Guard, said early Thursday that he could not confirm reports that it was headed to Arizona to be decommissioned.During Boone's news conference, he said that the nine people killed on board were all members of the 156th Air Wing of Puerto Rico's Air National Guard. An Air Force official told ABC News that five of the individuals were traveling as crew members on the flight and the other four were traveling as passengers.Boone said the cargo-plane was manufactured in the late 1970s and had been in Savannah for routine maintenance.The Puerto Rico Air National Guard is in the midst of getting newer C-130 aircraft to replace older ones that are being retired. The Puerto Rico National Guard has the oldest WC-130's in the Air Force.He said it was headed to Arizona on a "routine mission" when it plummeted onto busy Highway 21 about 11:30 a.m. Wednesday. That lengthy stretch of the highway remains completely closed this morning.An Air Force investigation is already underway but Dana White, chief spokeswoman at the Pentagon , said the Department of Defense will not be conducting a department-wide investigation.ABC News' Elizabeth McLaughlin contributed to this report.