Are "Gun-Free Zones" Dangerous?

John Lott has this interesting op-ed arguing that "gun-free zones" are actually more dangerous because they spawn attacks. He mentions that in my home town of Salt Lake City, the Trolley Square massacre (6 killed last year) took place at an area where the property owner had excluded firearms -- even those carried by concealed permit holders. He relies on anecdotal evidence for the argument, but seems to make a reasonable case for the proposition that "gun-free zones" are at greater risk of attack.