Story highlights Authorities won't disclose a motive for the bombing

Retired attorney Jon Setzer and his wife died in an explosion at their home

Their son-in-law is the sole suspect in the bombing, Tennessee authorities say

Ex-law partner: Setzer's health problems had made law work difficult for him

The son-in-law of a couple killed in a bombing at their rural Tennessee home has been charged with planting the deadly device, authorities announced Thursday.

Investigators arrested 49-year-old Richard Parker on two counts of felony first-degree murder and two counts of felony premeditated murder in connection with a package bomb that exploded at the rural Tennessee home of Jon and and Marion Setzer, investigators announced Thursday. Bond was set at $1 million, said Mark Gwyn, director of the Tennessee Bureau of Investigation.

Jon Setzer, a retired lawyer, died Monday when the package bomb detonated outside their home near Lebanon, about 30 miles east of Nashville. Marion Setzer died Wednesday evening at Vanderbilt Hospital. He was 74; she was 72.

Wilson County Sheriff Robert Bryan said Parker lived next door to his in-laws. Investigators would not discuss a motive and provided little detail about the case against Parker, but Gwyn said he is the sole suspect in the Setzers' deaths.

"Right now we feel like we have the person responsible for committing this crime in custody," he said.

Parker was convicted of arson in 1993, for which he served four years on probation, the TBI said.

Amid the debris, investigators found a note they said may have been attached to the bomb, but would not divulge its contents.

"This is a very important piece of evidence, because now you may have handwriting," said former ATF agent and bomb expert Joseph Vince.

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Authorities originally said they thought the bomb had been delivered by the U.S. Postal Service, but on Thursday they said they now believe that was not the case.

Officials said Setzer picked up the package from his mailbox, about 200 yards from the home. It detonated just inside the house, killing him and mortally wounding his wife.

"It doesn't make sense at all," family friend Ken Caldwell told CNN affiliate WTVF . "When I've heard it said that it was targeted, I thought, well, they must have targeted the wrong person."

Health problems

Before he retired, Jon Setzer worked on bankruptcy and other cases.

His former law partner, George Cate Jr., said Setzer was a dedicated servant and a pastor at "little country churches." The two met while serving in the Army Reserve.

Cate said he couldn't understand why anyone would want to target Setzer or his wife. "Nothing had happened in my recent times to make me anticipate anything of this kind happening," Cate told CNN affiliate WZTV

Cate and Setzer became partners at the law firm bearing their names from 1979 to 1991. Setzer worked on general civil cases and specialized in living trusts, his former partner said.

But health problems eventually made it difficult for Setzer to take care of all of his clients' needs, and he quit practicing, Cate said.

'A little anxious'

Neighbors said the blast has scared them; some told WZTV that officers checked other mailboxes on the street for similar devices.

"Of course, it makes us a little anxious to go check our own mailbox when we see something like this happen, because normally boxes are delivered and mail is delivered, and you don't question it," neighbor Tony Dedman told the affiliate.