8: In late June [2001], because of threats, Italy closed the airspace over Genoa and mounted antiaircraft batteries at the Genoa airport during the G-8 summit which President Bush attended” (258). We learn elsewhere that the Italians kept fighters in the air over the city, and that the threat was taken so seriously that Bush stayed overnight offshore, on an aircraft carrier. Although this example, like the first one, is about a threat in Europe, not the United States, it obviously counts against the thesis that there was a “failure of imagination” with regard to the possibility that terrorists might try to use airplanes to attack President Bush. (Another puzzling thing about this example is that the Commission, in mentioning that “antiaircraft batteries” had to be mounted at the Genoa airport, failed to point out that the White House and the Pentagon already have their own antiaircraft batteries, which would shoot down any aircraft except one with a transponder signal indicating that it belongs to the US military.)
Page 265-266
The 9/11 Commission Report: Omissions and Distortions
David Ray Griffin

As Dr Griffin points out, this isn’t about a threat in the US, nor is it necessarily about hijacking:

Italian officials say that the antiaircraft batteries they set up in Genoa were primarily intended to deter an attack from a small plane.
http://select.nytimes.com/search/restricted/article?res=F40715F8385E0C758EDDA00894D9404482

As such the example fails the tests he applied previously, although you’ll note he now changes this to a more general “thesis that there was a “failure of imagination” with regard to the possibility that terrorists might try to use airplanes to attack President Bush”. It’s an easier straw man to knock over, although this doesn’t help his case much. And it’s not the only problem here.

First, we don’t know that threats from the air were the specific reason that Bush stayed on a carrier. It was certainly reported that he did this to reduce his exposure to terrorist attack, but even if true, such attacks can come from many directions. And it’s worth noting that other leaders stayed offshore, too.

The official G8 Summit Web site said it was not so much violence by the demonstrators that they feared most, but "the possibility of a terrorist attack."

The head of Russia's Federal Bodyguard Service has warned of a plot by terrorist Osama bin Laden to assassinate George W. Bush at the summit and the U.S. President may be staying at U.S. Camp Darby military base in Livorno or offshore on the American aircraft carrier, USS Enterprise to avoid any terrorist risk.

The other leaders of the world's most industrialised nations -- the U.S., Britain, Canada, France, Germany, Italy, Japan, plus Russia -- are also staying offshore on a luxury cruise liner, the "European Vision, " chartered by the Italian government at a reported cost of $2.89 million.
http://edition.cnn.com/2001/WORLD/europe/07/17/genoa.security/

Certainly not everyone seemed to be taking all reports of the aerial threat seriously. Here’s a Time comment from June 2001:

For sheer diabolical genius (of the Hollywood variety), nothing came close to the reports that European security services are preparing to counter a Bin Laden attempt to assassinate President Bush at next month's G8 summit in Genoa, Italy. According to German intelligence sources, the plot involved Bin Laden paying German neo-Nazis to fly remote controlled-model aircraft packed with Semtex into the conference hall and blow the leaders of the industrialized world to smithereens. (Paging Jerry Bruckheimer…) The Russians, who believe a Bin Laden attack in Genoa is more likely to be carried out by their old enemy, the Chechens, have sent an advance team of anti terrorism experts (armed, we hope, with small-scale anti-aircraft weapons).
http://www.time.com/time/world/article/0,8599,131866,00.html

Presumably this was a different threat, as you wouldn’t put up antiaircraft batteries against “remote-controlled model aircraft”. Would you? Anyway, it illustrates the difficulties in picking out what’s real. And if we won’t know how seriously the US took these threats at the time, then we cannot say what impact they may (or should) have had elsewhere.

A side issue materialised post-911, in some odd apparent explanations of how these threats may have been discovered. It’s not clearly related to this issue, but if you’re interested then try this page.

In addition, Dr Griffin gives us comments about “antiaircraft batteries” at the Pentagon, including a basic description of how they work, yet mysteriously supplies no reference as to how he knows this. Read more on that here.