It is unclear whether Natalia is an only child, but in the post, Jeff referred to his daughter as "kiddo" and expressed pride in her skills with the weapon.
Despite a lack of evidence, claims that Rupnow was transgender have circulated online, a trend that has become common following school and mass shootings.
Just hours before Rupnow was named as the suspect in Monday's mass shooting in Madison, a social media user claimed to have spoken with the 15-year-old's alleged boyfriend.
"This remains an active and ongoing investigation," police said in a statement. "More information will be released as it is available. We currently need people to avoid the area."
Mangione was arrested at a McDonald's in Altoona, where he reportedly gave local police a fake ID and began trembling when questioned about recent travel to New York.