The victims were found in four homes in Tyrone, about 40 miles north of the Arkansas line. The 36-year-old gunman was discovered in a neighboring county, dead of what appeared to be a self-inflicted gunshot wound, Missouri Highway Patrol Sgt. Jeff Kinder said.
Kinder gave no information on a motive for the shootings or whether the gunman and the victims were connected. The names of the dead were being withheld until their relatives could be notified.
"This is a horrific tragedy, and our hearts go out to the victims of these senseless acts and their families," Gov. Jay Nixon said. He said crisis counseling will be made available to students and others.
All the victims were adults, Texas County Coroner Tom Whittaker told the St. Louis Post-Dispatch.
Whittaker told the newspaper that before the gunman was discovered dead, authorities found his mother dead of apparent natural causes on a couch at her home. She had been under a doctor's care and appeared to have been dead at least 24 hours, the coroner said.
Whittaker said investigators were still gathering evidence, but he speculated that the son "came home and found her deceased and then for whatever reason went on a rampage and started killing people."
The sheriff's office received a call about 10:15 p.m. Thursday from a young woman who said she had fled to a neighbor's home after hearing gunshots in her house, Kinder said. When officers arrived, they found two people dead.
Officers later found five more people dead and one wounded in three other homes. The wounded person was taken to a hospital; the victim's condition was not disclosed.
Tyrone is in largely rural Texas County, where the scenic rivers and woods draw canoeists, trout fishermen and deer hunters. The area has seen an exodus of shoe and garment factories over the decades, along with a drop in dairy and poultry farms, County Clerk Don Troutman said.
Troutman described Tyrone as little more than a pocket of houses. A couple of general stores are long gone, and the one-room schoolhouse has been converted to a community building, he said.
"There's not even a stop sign there," said Troutman, a lifelong Texas County resident who has been clerk for 36 years. He said of the bloodshed: "We've never had anything of this magnitude before. It's a shock."
Scott Dill, superintendent of the school district that serves Tyrone, called the area "bucolic" and beautiful, and added, "We are holding our breath as a community to find out specifics."
"We want to help people make sense of this tragedy," he said.
Whittaker told the Post-Dispatch that the discovery of the bodies over a several-hour span was numbing, considering that the county averages perhaps one homicide a year.
"At first I thought, 'I have three victims,' then we keep finding more victims," he said. "It's kind of like, 'Oh, gosh, what have we got here?' It's spread over miles."