LEBANON, Tenn. (CNN) -- Bewildered residents in rural Tennessee are grappling with fear and confusion as they try to understand why someone would send a bomb in the mail to their neighbor.
Retired lawyer John Setzer, 74, died Monday after "an unknown package exploded," the Tennessee Bureau of Investigation said.
His wife, 72-year-old Marion Setzer, was seriously injured and airlifted to Vanderbilt University Medical Center.
A barrage of federal, state and local authorities descended on the neighborhood near Lebanon, Tennessee, about 30 miles east of Nashville. The FBI, U.S. postal inspectors, the Department of Homeland Security and the Bureau of Alcohol, Tobacco, Firearms and Explosives are also investigating.
So far, they haven't publicly disclosed a motive. Those who knew the couple well are astonished.
"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."
A dedicated servant
John Setzer's 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 couldn't understand why anyone would want to target Setzer or his wife.
"I could hardly believe what I was being told because nothing had happened in my recent times to make me anticipate anything of this kind happening," Cate told CNN affiliate WZTV. "John and Marion Setzer are a great couple, and they have dedicated themselves to taking care of other people."
He remembered when the couple's son John was killed by a dog at age 3. After that, whenever the Setzers saw a news report about a child's death, they "would immediately go and visit with the parents of that child and share something of what they had gone through."
Cate and Setzer became partners at the law firm bearing their names between 1979 and 1991. Setzer worked on general civil cases and also specialized in living trusts, his former partner said.
Cate said Setzer's love of law became hampered by his health problems, which made it difficult to respond to all his clients' needs. Setzer continued working from home after leaving the office, but eventually decided to quit practicing, Cate said.
Terrified of the mailbox
On the Setzers' quiet rural street, neighbors were terrified about whether a bomb might arrive in their mailbox. Some told WZTV that officers went house-to-house Monday night to check mailboxes for 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.
Postal Inspectors have investigated an average of 16 mail bombs over the past few years, the U.S. Postal Inspection Service said. By contrast, the Postal Service has processed more than 170 billion pieces of mail each of the past few years.
The agency said mail bombs often have similar characteristics, such as a fake or non-existent return address. They often have excessive postage attached to the package because the sender doesn't want to deal face-to-face with a window clerk.
An $8,000 reward is available for information leading to the arrest and conviction of anyone responsible for the attack on the Setzers. Anyone with information can call 1-800-TBI-FIND.