The autopilot was engaged even after an alert warned the two pilots that the plane was dropping too quickly, investigators said Sunday at a news conference near the crash site.
"The autopilot was engaged until the last second of recorded data," Robert Sumwalt, a senior official with the National Transportation Safety Board, said.
Sumwalt said the auto throttle was also being used until seconds before the crash.
The Airbus A300 crashed at 4:45 a.m. Wednesday less than a mile from the runway at the Birmingham-Shuttlesworth International Airport.
Seven seconds before impact, the pilots received a sink rate alert, but investigators did not say whether pilots made any attempt to turn off the autopilot and regain altitude, the Los Angeles Times reported.
Sumwalt also said there were no problems with the controls on the plane, and no signs of engine failure.
Investigators said they are looking into how much the pilots slept before taking off from Louisville to Birmingham.
UPS has identified the victims of the crash as Cerea Beal Jr., 58, of Matthew, N.C., and Shanda Fanning, 37, of Lynchburg, Tenn.