By Tom Watkins and Holly Yan
The wife of the retired Tennessee lawyer who was killed this week when a mail bomb detonated outside their home has died, the Tennessee Bureau of Investigation said Thursday.
Marion Setzer, 72, died Wednesday evening at Vanderbilt Hospital.
Her husband, John Setzer, was killed Monday when the package exploded, shattering windows across the street from their rural Tennessee house.
Amid the debris, investigators found a note they said they believe was attached to the bomb; authorities 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.
Package was in mailbox
Officials said Setzer, 74, picked up the package on Monday from his mailbox, about 200 yards from the house. As he approached the front of the house, it detonated, killing him and initially critically 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."
Law enforcement sources said they're investigating how the package was delivered -- whether it came by commercial delivery or private carrier.
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 found in the home, including labels and pieces of paper, to determine whether they may have been part of the package or perhaps from previous deliveries from the same sender.
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 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.
In each of the past few years, the U.S. Postal Inspection Service has investigated an average of 16 mail bombs from among the more than 170 billion pieces of mail the postal service typically processes, it said.
The agency said mail bombs often bear fake or non-existent return addresses and carry excessive postage, presumably so that the sender does not have to interact with a window clerk.
An $8,000 reward has been offered for information leading to the arrest and conviction of anyone responsible for the attack.
CNN's Evan Perez, Brian Todd, Dave Alsup and Michael Martinez contributed to this report.
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