Thirteen weapons were recovered, six from the school and seven from Mercer's residence.
All of the weapons were purchased legally and traced to a federal firearms dealer.
Seven of the weapons were purchased by the shooter or his family members in the last three years.
According to the Douglas County Sheriff's Office, the weapons recovered were pistols, rifles and a shotgun.
Federal law prohibits felons, drug users, domestic violence misdemeanors and people involuntarily committed to mental institutions from possessing firearms.
Oregon recently tightened gun restrictions in the wake of other shootings in the state.
In 2012, a gunman stormed a shopping center in Portland with a stolen AR-15 semi-automatic rifle, killing two people before taking his own life.
Family members of those victims backed a measure that went into effect in August, requiring background checks on most private gun sales.
Only 32 percent of Americans own a firearm or live with someone who does.

How do we arm the other 68%?