Virginia governor Timothy M Kaine has tightened the state's gun laws in response to the killing of 32 people at Virginia Tech earlier this month.
Mr Kaine's order clarifies who should be barred from buying firearms
Mr Kaine issued an executive order banning people found to be dangerous and ordered to have involuntary mental health treatment from purchasing guns.
The gunman, Cho Seung-hui, was able to buy weapons despite having a history of mental illness.
The 23-year-old South Korean student took his own life after the tragedy.
The shootings began when two people were killed at the West Ambler Johnston Hall, a university dormitory, on the morning of 16 April.
Two hours later Cho killed 30 students and teachers at the Norris Hall complex across campus.
No motive has yet been established for the killings.
Absent from database
In a statement, Mr Kaine said the database of people barred from purchasing firearms should "include any determination that someone is mentally ill and so dangerous to himself or others as to warrant involuntary treatment".
The executive order also clarifies that there will be no distinction on the database between individuals ordered to undergo inpatient or outpatient treatment.
An existing loophole meant Cho was not entered onto the database even when a Virginia judge ruled he was a danger to himself, because he was treated as an outpatient and never committed to hospital.
The Virginia State Police have now been directed to request copies of orders both for involuntary inpatient and involuntary outpatient care from district courts.