The boy was being watched by a relative at home in Oro Grande, California, when the pit bull attacked the child and "would not let go," authorities said.
The neighbor fired his crossbow that hit a pit bull, then went through a door, and killed the man he was trying to save, authorities said.
The girl, named Aria, was with her mother at their home in Phoenix, Arizona, with several other people when the dog attacked her.
"Dogfighting is a blight on humanity, one that has no place in the Northern District of Florida or anywhere else," Lawrence Keefe, the United States attorney for the Northern District of Florida, said in a press release.
The suspect is now in custody over the gun mishap in Detroit, Michigan, after calling the police on himself.
"All I could do was stand there and watch my dog get eat," said Cynthia Robinson, the owner of Tank, a 6-year-old pit bull which was attacked by the gator.
Earl LaFrance was identified as a potential suspect after being linked to a previous complaint alleging that a dog was being kept inside a garage.
The dog—reportedly a pit bull terrier—bit a number of children after it got loose during the recess of third and fourth graders.
When approached, the dog was "barking and lunging on the leash."
Police said the 8-year-old pit bull raced toward officers "like a missile" before they fired shots.