In this Aug. 1, 1966 file photo, smoke rises from a sniper's gun as he fires from the tower of the University of Texas administration building in Austin, Texas at people below. Grant Duwe, a criminologist with the Minnesota Department of Corrections, says, "Mass shootings provoke instant debates about violence and guns and mental health and that's been the case since Charles Whitman climbed the tower at the University of Texas in 1966."—Photo by AP

A message is left on a door in remembrance of those killed down the street from the Sandy Hook School in Newtown, Connecticut. Twenty-six people were shot dead, including twenty children, after a gunman identified as Adam Lanza opened fire at Sandy Hook Elementary School.—Photo by AFP

Cindy Sparr boxes a AK-47 style rifle purchased by a customer at Freddie Bear Sports sporting goods store in Tinley Park, Illinois. Americans purchased a record number of guns in 2012 and gun makers have reported a record high in demand.—Photo by AFP

A weapons dealer cleans his AK-47 assault rifle at his house in Laghman province. Arms purchases are soaring in Afghanistan, along with the price of weapons, a sign that many Afghans fear a return of the Taliban, civil war or rising lawlessness.—Photo by Reuters

Jack Gilchrist, 10, chews on a candy cane as he holds a 'Keep guns out of my school' sign as he and his mother protest outside the Breslin student center in East Lansing, Michigan.—Photo by Reuters

Jason Zielinski shows a customer a selection of AR-15 style rifles being offered for sale at Freddie Bear Sports sporting goods store in Tinley Park, Illinois. About 47 per cent of Americans own guns.—Photo by AFP

Members of Sisters of Christian Charity walk to lay flowers in front of the Sandy Hook School in Newtown, Connecticut.—Photo by AFP

A weapons dealer cleans his AK-47 assault rifle at his house in Laghman province. Arms purchases are soaring in Afghanistan, along with the price of weapons, a sign that many Afghans fear a return of the Taliban, civil war or rising lawlessness.—Photo by Reuters

Teddy bears, flowers and candles in memory of those killed, are left at a memorial down the street from the Sandy Hook School in Newtown, Connecticut.—Photo by AFP

San Francisco police officers document guns that are being surrendered during a gun buy back program in San Francisco, California. The San Francisco police department held a one-day gun buy back event that paid $200 per gun turned in.—Photo by AFP