The only people who know for certain what happened inside a small-town post office in West Tennessee are the two workers who were shot and killed and whoever pulled the trigger.

Federal, state and local authorities don't know who did it _ or why _ and pleaded for the public's help Tuesday for any clues that might help solve the case, offering a $50,000 reward.

The investigative team of more than 60 federal, state and local law enforcement officials have provided little information except that the attack happened soon after the post office opened for business Monday. Several two-door burgundy cars were stopped and searched Tuesday, their drivers learning from investigators that such a vehicle was involved in the shooting.

Yellow crime-scene tape roped off the one-story brick post office, and two large police command posts were set up in Henning, about 45 miles northeast of Memphis. A truck hoisted an investigator to the roof, where he combed for clues. Outside the post office, the flag had been lowered to half-staff Tuesday. Authorities haven't announced a motive or arrests, but any suspects would face federal charges because the victims were U.S. government employees.

The Lauderdale County district attorney, Mike Dunavant, has said little except that the killings were done with "disturbing violence."

Killed were Judy Spray, 59, a rural carrier associate, and Paula Robinson, 33, a retail clerk. Their bodies were removed from the post office and sent to a medical examiner late Monday for autopsies.

Left with little information, residents constructed their own theories _ from robbery to a drug package delivery gone bad, said farmer George Arender, who was filling up his red pickup truck Tuesday at the town gas station.

"They're devastated," Arender said. "They just can't figure out a motive for this."

The women were fixtures at the post office, well-known to residents who often come to pick up their mail. The post office sits between a self-service car wash and a coin-operated laundry called "Mom's" in the town of about 1,200 people.

Many residents recalled that they would walk in to buy stamps and see Robinson's smile. Another local woman said she knew Spray, describing her as being "nice as can be" when she delivered the mail.

"When we were outside, she would wave and smile at us," Wendy Willis said.

Family members have declined comment.

Yulanda Burns, a spokeswoman for the U.S. Postal Inspection Service, would not say whether there was working video surveillance in the post office. But she said rumors that a fake video camera had been installed were untrue.

The getaway car's description has not been released, though one woman said Tuesday that her car was searched by investigators looking for a similar vehicle.

Student Cortesha Foster, 25, said a friend called her to say police searched her burgundy two-door car as part of their investigation. The friend recommended that Foster, who drives a Chevrolet Monte Carlo of the same color, go to the crime scene.

"They told me to get out. Searched my vehicle. Searched my trunk, and everything in there," she said. She was allowed to leave afterward.

Postal inspectors in blue vests also briefly searched the car wash next door as rain drizzled from an overcast sky.

The post office is less than a half-mile away from the museum dedicated to "Roots" author Alex Haley, who died in 1992. The 1976 book won a Pulitzer Prize and was the basis for a top-rated TV series.

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Associated Press writers Lucas L. Johnson II and Kristin M. Hall in Nashville contributed to this report.

(This version CORRECTS Spray's age to 59 instead of 58, per new information from Postal Inspection Service.)