(CNN) -- A week after Adam Lanza opened fire on Sandy Hook Elementary school, murdering 20 children, six school employees and his own mother before taking his life, the nation is left asking, "Why?"
On response is to explore Lanza's mental health. While CNN has not confirmed that Lanza was mentally ill, mental illness has been a common thread in many of the mass shootings in the last 60 years.
Here's a look -- by the numbers -- at guns and mental health:
310 million - Total number of non-military firearms in the U.S.
37 - Percentage of Americans who supported major restrictions on gun ownership according to a CNN/ORC poll from December 17.
15 - Percentage of Americans who felt all guns should be illegal according to the same CNN poll.
47 - Percentage of Americans who supported major restrictions or felt all guns should be illegal according to an earlier CNN poll from August.
47 - Percentage of Americans who told Gallup in 2011 that they had at least one gun at home.
At least $1,500 - Retail price of a "higher-end" shotgun or rifle.
16,799 - Homicides in the United States in 2009.
11,493 - Homicides by firearm in 2009.
26.2 - Percent of adults in the U.S. who "suffer from a diagnosable mental disorder in a given year," according to the National Institute of Mental Health.
6 - Percent of U.S. adults who have a serious mental illness.
3.4 - Percentage of non-institutionalized adults with "serious psychological distress" in the past month, according to the 2011 data from the CDC.
62 - Mass shootings in the United States since 1982.
49 - Cases where the shooter obtained the murder weapon legally.
68 - Total of semiautomatic weapons used by the shooters, from of a total of 142 firearms.
24 - Percentage of Americans who said in 2011 they felt that mass shootings could be prevented by stricter gun control laws.
44 - States (plus District of Columbia) with laws regarding the ability of a mentally ill person to possess a firearm.
15 - Percent of those who felt that a good preventative measure would be better mental health screenings.
23 - States (plus Washington, D.C.) that have submitted fewer than 100 mental health records to the National Instant Criminal Background Check System since it was created in 1999.
186,068 - Background checks conducted in Connecticut in 2011 using the NICS system.
6,103 - Potential gun sales in 2010 rejected after a background check indicated mental health issues.
58 - Percentage polled in 2011 after the shootings in Tucson, Arizona, that felt mass shootings were mainly isolated events carried out by "troubled individuals."
44 - Percentage of those who felt that way after the Sandy Hook shootings.