(Reuters) - Authorities continued to search for answers on Saturday in the murders of seven people in a small Missouri town as details emerged of the alleged shooter's criminal background and local media reported the identities of another three victims.

Joseph Aldridge, 36, embarked on a shooting rampage late on Thursday in Tyrone, Missouri, killing seven relatives and neighbors before fatally shooting himself, police said.

Aldridge had drug issues. In 2008, he was sentenced to 21 months in federal prison for possessing a handgun while being a user of marijuana, and was ordered to undergo mental health and substance abuse counseling, court records show.

Aldridge went to several houses during his rampage and most of the victims were found in their bedrooms, according Texas County Coroner Tom Whittaker.

The Missouri State Highway Patrol had announced victims Garold Aldridge, 52, his wife Julie Aldridge, 47, Harold Aldridge, 50, and his wife Janell Aldridge, 48.

The Kansas City Star and St. Louis Post-Dispatch reported that Aldridge had also killed Carey Shriver and his wife Valirea Shriver, and Carey's father Darrell Shriver. When contacted by Reuters on Saturday, the highway patrol declined to verify the identities of the three additional victims.

Darrell's wife Martha was wounded but survived, the media reports said. The Shrivers belong to a prominent family that has lived in the area for generations and operates several businesses, including a cabinet making shop, a cattle ranch and an auto dealership.

Whittaker said Aldridge may have been triggered to carry out his killing spree by the discovery of the body his 74-year-old mother who had apparently died of natural causes, but that the authorities were considering several other possible explanations.

"We have some ideas of what may have been going on, but why it went to this extent, we don’t know and I don’t know that we ever will," Whittaker said.

Whittaker said the body of Aldridge's mother had no signs of trauma and an autopsy on Saturday is intended to rule out homicide. She apparently had been ill and may have been dead for up to 24 hours, he said.

There was no sign of forced entry in any of the houses where the shootings occurred, said Texas County Sheriff James Sigman, adding that the killings had shattered a sense of safety in the town. "Start locking your doors. The world is changing. You got to be safe," he said.

Tyrone, with about 50 residents, is about 160 miles southwest of St. Louis, near the Mark Twain National Forest, in an area that attracts hunters, campers, and river rafters.

(Reporting by Kevin Murphy in Kansas City; Additional repoting by Carey Gillam in Kansas City; Editing by Raissa Kasolowsky)