In the vast majority of mass shootings, the shooter has later been found to be mentally ill and used a legally purchased firearm to carry out the shooting.
Current federal firearm regulations forbid the sale of firearms to those with a history of mental illness. Current regulations have FAILED to prevent the sale of firearms to the mentally ill, and these firearms are being used in mass shootings.
Federal law should require that before ANY firearm purchase can be finalized, the purchaser must submit to an independent mental health evaluation by a qualified professional and provide the results of this evaluation to the seller of the firearm before the purchase can be finalized. This may not prevent all mass shootings, but it will at least introduce one "inspection" step in the process.