Now, this is kind of bizarre: The Arizona Republic reports that the FBI foiled a plot to do some serious damage to the Intel plant in Chandler, Ariz. Apparently, an ex-employee who was arguing with Intel over disability payments planned to seek into Intel's plant there, turn on the gas, and shoot the place up.
The ex-employee, a guy named David Dugan, was, according to the paper diagnosed with a bi-polar disorder and had tried to commit suicide. His medical condition notwithstanding, this being America, he was able to buy an AK-47 semi-automatic rifle. Let's pick it up at this point from the Republic's report:

The [criminal] complaint [filed in court] then says ... police and a friend of Dugan "removed a large amount of ammunition from Dugan's home" ...., but Dugan refused to turn over numerous firearms and other weapons in his home, which he also monitored with cameras and a surveillance system.
On Oct. 20, Dugan obtained the weapon, 1,000 rounds of ammunition and five magazines for the AK-47 from the firearms store, the complaint says, and then told an Intel employee via telephone about the purchase.

Are there no cooling-off periods or even the slightest background checks for those in the U.S. who want to buy semi-automatic weapons?