The shooting happened Monday morning at the one-story, brick building in Henning, the Lauderdale County Sheriff's Department said. District Attorney Mike Dunavant said Monday afternoon that authorities were looking for a vehicle but would not elaborate except to say the case involved "disturbing violence."
No arrests were made, and postal officials pleaded with the public to come forward with information about the crime.
The victims were identified by authorities as Judy Spray, 58, a rural carrier associate, and Paula Robinson, 33, a retail clerk.
The post office, which sits between a self-service car wash and a coin-operated laundry called "Mom's" in this town of about 1,200 people, often has residents coming in to pick up their mail. Home delivery isn't provided in Henning, some 45 miles northeast of Memphis.
Beth Barnett, a spokeswoman for the U.S. Postal Service, said that five people usually work in the post office but that she was not sure how many were there at the time of the attack. Post offices are not immune to crime, but robberies at post offices are uncommon, she said.
Mary Hammock, who works at a nearby market, said Monday afternoon that she had been in the post office about 8:25 a.m. and noticed it was not as loud or busy as normal.
"I knew something didn't feel right because it was real quiet," she said. She returned to the market and heard police sirens about 15 minutes later.
"I might have been real close probably to losing my life," she said.
Around midday, plainclothes investigators were scanning the area along a railroad track that sits behind the post office. Lines of yellow police tape kept people away from the building as a crowd gathered nearby, some sitting in chairs, waiting for more information about what happened.
Crime scene investigation trucks were parked outside, including one from the Tennessee Bureau of Investigation.
Ella Holloway, who lives within walking distance of the post office, said she knew one of the women killed, later identified as Robinson. Holloway said she would be greeted by the woman's smile when she went to the post office to buy stamps. Another local woman said she knew Spray, describing her as "nice as can be."
"When we were outside, she would wave and smile at us," Wendy Willis said.
Tony Burns, a state employee at the Tennessee Capitol in Nashville, said his sister-in-law is a postal service worker who was assigned to the Henning office Monday. She told him that the shooting happened during a robbery attempt, but that she escaped unharmed. The sheriff's department also described the incident as a robbery.
Standing on a street corner near the post office, city resident Emmitt Hennings, a 71-year-old retiree, said it was hard to comprehend what happened.
"I just couldn't believe it, not in this town," Hennings said. "It's too quiet."
Postal officials offered a $25,000 reward for information leading to an arrest and conviction in the case.
A forensic team remained in the post office late Monday afternoon. The bodies were to be taken to Memphis for autopsies.
The post office is less than a half-mile away from the museum dedicated to the "Roots" author Haley, who died in 1992. The 1976 book won a Pulitzer Prize and was the basis for a top-rated TV series. The story chronicled his family history from Africa to slavery and freedom in the U.S., and it inspired many people to research their own families' roots.
Associated Press writers Lucas L. Johnson II and Kristin M. Hall in Nashville contributed to this report.