Seven people were murdered Thursday night in a house-to-house rampage in the small Missouri town of Tyrone before their suspected killer was discovered dead of an apparent self-inflicted gunshot wound, police said.

A ninth person found dead in connection with the investigation is believed to have died from "natural causes," according to authorities.

"This is a horrific tragedy, and our hearts go out to the victims of these senseless acts and their families," Gov. Jay Nixon said.

Officers were called to a home Thursday at 10:15 p.m. by a teenage girl who reported hearing gunshots inside a Tyrone residence. The girl fled to a neighbor's house to call authorities, and police responding to the scene found two people dead, according to officials.

Upon further investigation, authorities found five additional victims who were dead and one victim who was wounded in three additional residences, Sgt. Jeff Kinder of the Missouri Highway Patrol said. All the homes involved in the attacks are in Tyrone, an unincorporated community in Texas County, Mo., about 140 miles southwest of St. Louis and about 40 miles north of the Arkansas line.

A search of another residence revealed the body of a deceased elderly female who appeared to have died from natural causes, officials said.

The Houston Herald newspaper, citing a law enforcement official, reported that the woman was the mother of the suspected shooter.

Texas County Coroner Tom Whittaker told the St. Louis Post-Dispatch that the woman was found dead on a couch at her home. She had been under a doctor's care and appeared to have been dead at least 24 hours, he said.

Kinder told reporters at a press conference Friday morning that investigators are not considering the woman to be a victim at this time.

The suspected gunman, a 36-year-old man from Tyrone, was found dead of an apparent self-inflicted gunshot wound in a parked vehicle in Shannon County, authorities 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."

Kinder said authorities were investigating six scenes in total Friday -- including the spot where the suspected gunman shot himself.He said police will not release the names of the victims and the suspect until next of kin are notified.

All the victims in the shooting were adults, according to Whittaker.The injured victim is being treated at a nearby hospital.

"In our job we see a lot of bad stuff and this is bad," Kinder said.

A neighbor of one of the crime scenes told the Houston Herald that police came to the door at 3:45 a.m. Friday to check for victims. Authorities told residents in the area to stay in their homes and not to open their doors to strangers. The paper said the Houston School District has informed staff to arrive early work for counseling sessions.

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."

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The Associated Press contributed to this report.