(CNN) An arrest has been made in an explosion that killed two people near Lebanon, TN. Retired attorney John Setzer was killed Monday, and his wife Marion died Wednesday. Investigators say Richard Parker, the Setzers’ son-in-law, placed a package outside the Setzers’ home and it exploded. Parker is being held on a $1 million bond. 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 the package was not mailed, but was placed there by Parker. State and local authorities descended on the neighborhood near Lebanon, Tennessee, about 30 miles east of Nashville. Health problems 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.