In her statement Amanda Rodrigues admitted to having an argument with her husband in which they came to blows but denied killing him.
She said she found her husband dead the next morning in the two-story apartment.
The former boxer was under the influence of alcohol at the time of his death, the investigation found.
Gatti was the International Boxing Federation super-featherweight world champion from 1995 to 1997. In 2004, he won the World Boxing Council world title at junior-welterweight but lost it a year and a half later to Floyd Mayweather Jr. He retired in 2007.
Gatti fought 49 times as a professional, winning 40 of his bouts.