Investigators say the suspect's parents felt the 28-year-old should not own weapons, and did not realise the guns had been concealed in their house.
Six people, including three children age nine, were killed in the attack at the Covenant School.
The suspect was under "doctor's care for an emotional disorder", police say.
And it was the school that was targeted rather than any particular individual, officers revealed.
Tennessee has no laws that allow police to seize guns from violent suspects.
Despite the absence of such so-called red-flag laws, police said they would still have sought to have the weapons confiscated if authorities had had any warning that the suspect could have posed a threat.