How did they get their guns?

http://www.nytimes.com/interactive/2015/10/03/us/how-mass-shooters-got-their-guns.html?_r=0


When you read the article, most of these active shooters purchased their guns legally, despite the fact that some of them had mental illnesses.  My question would be, how thorough are these background checks? I just purchased  my first gun in Virginia, and though I don't have a criminal background, it took approximately 10 minutes for them to get back with me.  I thought it was funny when my wife was asking about purchasing a weapon, she was told she couldn't.  For one reason, she isn't a Virginia resident, despite her being on active duty military orders.  She either had to be a Virginia resident or her orders had to show she is stationed in Virginia.  My next question is, what disqualifies a person from purchasing a weapon?  When the background checks are conducted, what are they looking for that disqualifies someone.  One individual had a drug charge and another had an assault charges, and they were still able to get a gun.  I am a gun owner, and I enjoy going to shooting ranges, but for shooters who got the gun from their parents, I believe there needs to be some serious gun control, locked up and hide the key.  It's pretty much impossible to determine if a person you pass by or even someone you see regularly will open fire in the crowd.  As inconvenient as it may be for a lot of gun lovers, I think it would be wise to have stricter rules when purchasing a gun.