Robert Sumwalt of the National Transportation Safety Board said the plane's black box recorder had not yet been recovered, Al.com reported. At a news conference, he said there was no sign the pilots had communicated any difficulties to air traffic controllers.
Both pilots were killed when the plane, en route from Louisville, Ky., crashed as it approached Birmingham-Shuttlesworth International Airport at about 5 a.m.
One pilot was identified by her family as Shanda Carney Fanning, 37, of Lynchburg, Tenn. Sumwalt said the other pilot's identity would be released by local officials.
The tail of the downed plane was still smoldering hours after the crash. Sumwalt said NTSB investigators, who were on the scene 5 hours after the crash, were focused on collecting perishable evidence.
Debris from the plane was scattered across a residential neighborhood. Sumwalt asked the public to report anything they saw immediately.
Witnesses reported hearing a loud boom followed by the noise of other explosions. Most of the plane came down in a field.