(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 Jon 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.
Law enforcement sources say that a note was recovered from the bombing scene and was believed to have been attached to the bomb.
Investigators on Wednesday were cataloging evidence of a blast that was powerful enough to extend through much of the house and destroy windows.
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."
Michael Knight, spokesman for the ATF in Nashville, said investigators are looking at how the package was delivered and whether there were any threats to the family. They haven't identified suspects, he said.
Investigators are testing items in the home, including labels and pieces of paper, to determine whether they were part of the package or perhaps previous deliveries.
One bomb expert, Joseph Vince, a former ATF agent, said investigators will looking at whether revenge may be a motive.
"Clients that didn't like the way they were represented -- or they represented some other client -- now this is payback," said Vince, director of criminal justice programs at Mount St. Mary's University in Maryland.
"With a note in there, it very well may be some intentional revenge or something, that the attacker, the bomber, wanted to send them a message," Vince added. "This is a very important piece of evidence, because now you may have handwriting."
The victims took the package from the mailbox about 200 yards away from the house, which is in a rural area, Knight said. The blast occurred near the front door of the house, though it's not clear whether just inside or right at the door.
Authorities said they haven't seen any copy-cat incidents or secondary suspicious packages, which often happens in these cases.
A dedicated servant
Before he retired, John 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 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.
CNN's Evan Perez and Michael Martinez contributed to this report.