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.
[...]
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.
[...]
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.