Congress on Tuesday observed a moment of silence for the 12 victims killed by Aaron Alexis at the U.S. Navy Yard in Washington, D.C., the day before.

In commemorating those killed -- Michael Arnold, Martin Bodrog, Arthur Daniels, Sylvia Frasier, Kathleen Gaarde, John Roger Johnson, Mary Francis Knight, Frank Kohler, Vishnu Pandit, Kenneth Bernard Proctor, Gerald L. Read and Richard Michael Ridgell -- Congress members did what they had done many times before following a mass shooting.

Then they got back to business as usual. What they didn't do was begin any kind of debate about gun safety legislation -- specifically universal background checks, something 90 percent of Americans and 60 percent of gun owners support.

But this latest mass shooting shows that gun laws do have some impact. Alexis couldn't buy the AR-15 assault rifle that was his first choice in the Virginia gun store where he shopped shortly before the shooting because he didn't have a Virginia driver's license.

But there was a loophole. That same out-of-state buyer prohibition wasn't an issue on the shotgun that he ultimately bought at the store: the same shotgun he used to shoot his way into the building where he killed 12 people. While existing federal law does bar the mentally ill from legally buying guns from licensed dealers, that law only applies to people involuntarily committed to a mental health facility or declared mentally ill by a judge. Neither of those things had happened with Alexis, though he claimed to be bombarded by microwaves.

But there are much bigger loopholes pointing up the need for universal background checks and toughened gun safety laws. A report by the Mayors Against Illegal Guns organization released Wednesday found that many people, including criminals, exploit "private sale" loopholes in our existing law.

Looking at just one of countless online gun sale sites, Armslist.com, the group's analysis found that individuals looking for guns there are nearly four times more likely to have prohibiting criminal records than buyers at licensed dealers.

Where fewer than 1 in 100 prospective buyers at a dealer fails a federal background check due to a criminal history, 1 in 30 prospective buyers on Armslist would be prohibited for that reason, yet there is no real background check that prevents them from completing the online sale.

Armslist, with tens of thousands of gun ads from every state, is a large, national online marketplace where private sellers and buyers exchange guns. The majority of ads are posted by sellers, but would-be buyers also post "want-to-buy" ads. To learn more about would-be gun buyers online, the mayors group reviewed the identifying information voluntarily provided by would-be gun buyers in those Armslist want-to-buy ads. They found that one in 30 had committed crimes that prohibited them from possessing a firearm. But remember, the mayors were the only folks checking because private sales don't fall under existing law.

"To put this number in context, if one in 30 people on a Boeing 747 were on a terrorist watch list, the plane would have 22 suspected terrorists aboard," according to the mayors' report, titled "Felon Seeks Firearm, No Strings Attached.

The National Rifle Association once supported the broader use of the National Instant Criminal Background Check System, dubbed NICS. But as the NRA has increasing become a gun-sales advocate rather than a gun-owners advocate, it now opposes efforts to close this growing "private seller" loophole. The excuse? Criminals won't submit to background checks.

"This report demonstrates that their claim, is both false and true," the mayors report states. "Criminals undeniably do submit to background checks: In 2010 alone, federal and state checks blocked more than 150,000 gun sales to prohibited buyers. But criminals also undeniably avoid background checks -- by exploiting the private sale loophole."

Lawmakers both fear the NRA and lust for its campaign donations and support; so much so that they can't hear 90 percent of America.

Let's all have a moment of silence. It seems that's all we can do.