NASHVILLE, Tenn. (AP) — A three-day investigation of an explosion that killed a widely loved and well-respected Tennessee couple in their 70s ended Thursday with authorities charging a son-in-law who lived directly behind them and had been previously convicted of arson.

Richard Parker was charged with first-degree murder in the deaths of Jon and Marion Setzer, as well as unlawful possession of a prohibited weapon, said State Fire Marshal's Office spokeswoman Katelyn Abernathy.

Abernathy said she did not have any information about a possible motive for the bombing. Authorities did not release any information about the Setzers' daughter, Parker's wife, other than to say that he was the only person charged.

Jon Setzer, 74, was an attorney who handled wills and trusts, but he had been in very ill health in recent years. Friends said he was on dialysis and had heart problems and high blood pressure, among other health issues.

Marion Setzer, 72, had formerly worked as a dental hygienist.

A package exploded at their home on Monday at about 5 p.m., killing Jon Setzer immediately and critically injuring Marion Setzer, who died at a hospital on Wednesday.

Reached by telephone the day before his arrest, Parker declined to talk about the deaths with The Associated Press. Parker ran Legacy Restorations, a business that specializes in historic restorations, according to its website. His house was just behind the Setzers' in a semi-rural area of Lebanon, about 40 minutes east of Nashville.

Parker was convicted of arson in 1993 in Giles County and sentenced to four months of probation, according to records.

The Setzers were well-loved and respected by their former colleagues and neighbors, who were struggling to comprehend their deaths.

"We are just dazed by what happened," Nashville attorney John Stark said. "Jon was one of the good guys. He was a good lawyer. He taught Sunday school."

Stark, who said he's known the Setzers for more than 30 years and attended church with them, described the former lawyer as quiet and humble man.

John Lloyd, a retired dentist, said he also has known the family for years, first when Marion Setzer worked for him as a hygienist in Nashville and later when they attended church together in Lebanon.

"They were two of the finest people I ever knew, good Christian people who loved their children," Lloyd said.

Lloyd said Marion Setzer stopped working for him when she became pregnant with the couple's son, Jon Leo Setzer Jr. The child died at age 3 when he was mauled by a neighbor's German shepherd in 1977.

The story was front-page news at the time as officials debated whether to have the dog put down.

Bob Taylor, who lived about a block from the Setzers for many years, said they were "nice folks" and good neighbors. Jon Setzer volunteered do the legal work to set up their local homeowners association. Taylor said he and his young children all helped search for the Setzers' little boy when he went missing. Taylor and his wife had not heard from the Setzers for a few years before they learned about the explosion on television.

"My wife was home by herself," he said. "It just knocked her for a loop."