The nine-car train, traveling from New Orleans to Chicago, derailed about 25 miles north of Jackson Tuesday, authorities said, leaving in its wake twisted and heavily damaged track.
"We have one confirmed dead," said Amy Carruth, a spokeswoman for the Mississippi Emergency Management Agency in Jackson. "We understand some of the injured are possibly critical."
All passengers and crew were accounted for late Tuesday, Carruth said.
Lee Stokes, also of MEMA, said that while the derailment was believed to be an accident, the FBI had sent agents to the scene. Gov. Haley Barbour declared a state of emergency.
Coroner Ricky Shivers identified the dead passenger as Clara L. Downs, 68, of Chicago. Stokes said it appeared about 65 people suffered "minor to critical injuries."
The train appeared to leave the track on a trestle five or six feet over a swampy area in Mississippi's Delta region.
The train consisted of one engine, one baggage car, seven passenger cars and one unoccupied passenger car. Amtrak, which runs national train services, said all nine cars derailed, with the first seven coming to rest on their sides. The locomotive remained on the track.
Dan Stessel, a spokesman for Amtrak, said 68 passengers and 12 crew members were on the train, the City of New Orleans. He said he had no information on what caused the accident.
Amtrak has sent passenger assistance teams to the scene of the accident and has established a toll-free number with information for friends and relatives of passengers who were on the train: 1-800-523-9101.
Two passengers, Shelia Doyle and her husband Charles Lotz, told The Clarion-Ledger newspaper that they saw a cloud of dust and the train started tipping on its side. "Oh, my God, we are derailing," Lotz said.
Authorities said many rescuers were forced to use off-road vehicles to reach the scene because of woods and soggy ground. Searchers used flash lights and portable lighting during the night as they moved from car to car checking for more injured.
Stessel said the train made several stops after leaving New Orleans about 1:55 p.m., including Jackson. He said the train derailed at about 7 p.m., near the Yazoo-Madison county line, before its scheduled stop in Yazoo City.