The Violent Crime Control and Law Enforcement Act, enacted in 1994, included the Federal Assault Weapons Ban, and was a response to public fears over mass shootings. This provision prohibited the manufacture and importation of some firearms with certain features such as a folding stock, pistol grip, flash suppressor, and magazines holding more than ten rounds. A grandfather clause was included that allowed firearms manufactured before 1994 to remain legal. A short-term evaluation by University of Pennsylvania criminologists Christopher S. Koper and Jeffrey A. Roth did not find any clear impact of this legislation on gun violence. Given the short study time period of the evaluation, the National Academy of Sciences advised caution in drawing any conclusions. In September 2004, the assault weapon ban expired, with its sunset clause.