LEBANON, Tenn. (CNN) -- The wife of the retired Tennessee lawyer who was killed this week when a mail bomb detonated outside their home died Wednesday, the Tennessee Bureau of Investigation said Thursday.
Marion Setzer, 72, died at Vanderbilt Hospital, the agency said.
Her husband, John Setzer, was killed Monday when the package exploded, shattering windows across his rural Tennessee house.
Amid the debris, investigators found a note they said they believe was attached to the bomb.
Authorities would not reveal what the note said.
"This is a very important piece of evidence, because now you may have handwriting," said former ATF agent and bomb expert Joseph Vince.
Officials said Setzer, 74, picked up the package Monday from his mailbox, about 200 yards from the house. It exploded near the front the house, killing him and critically injuring his wife.
Law enforcement sources said they're investigating how the package was delivered -- whether it came by commercial delivery or private carrier.
"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."
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.
Officials 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.
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.
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.
'A little anxious'
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.