The story...

False blips, or radar injects, were placed on FAA radar screens on the morning of 9/11, as part of a military exercise. This confused and delayed any response to the hijackings, ensuring they would succeed.

Our take...

The claim of false blips on FAA radar is rarely questioned, most people seem to assume it's true. And yet, there's little data ever provided to back it up. So what is the evidence?

1. Northern Vigilance

A military exercise called Northern Vigilance was being conducted on September 11th. Canada's Toronto Star carried this report.

"Deep inside a mountain in Colorado and far beneath the granite of North Bay, members of the North American Aerospace Defence Command (NORAD) are at full "battle staff" levels for a major annual exercise that tests every facet of the organization.

Operation Northern Vigilance, planned months in advance, involves deploying fighter jets to locations in Alaska and northern Canada. Part of this exercise is pure simulation, but part is real world: NORAD is keeping a close eye on the Russians, who have dispatched long-range bombers to their own high north on a similar exercise.

Everything is going as planned when Capt. Mike Jellinek arrives for his 6 a.m. shift. The Canadian will be overseeing the crew staffing a crucial post inside the mountain- NORAD's command centre.

Whether it's a simulation or a real-world event, the role of the centre is to fuse every critical piece of information NORAD has into a concise and crystalline snapshot.

An hour into his shift, something unscripted happens. NORAD's Northeast Air Defence Sector (NEADS), based in Rome, N.Y., contacts the mountain.

The Federal Aviation Administration has evidence of a hijacking and is asking for NORAD support. This is not part of the exercise.

In a flash, Operation Northern Vigilance is called off. Any simulated information, what's known as an "inject," is purged from the screens".
http://www.911readingroom.org/bib/whole_document.php?article_id=92

The final sentence is the one that's referred to most often. In Crossing the Rubicon, Mike Ruppert said:

"The reference to “injects” was chilling. No other mainstream press (especially in the US) had mentioned that false radar blips had been inserted onto radar screens on September 11th. But on whose screens? Where?".

Well, the article is clearly set in NORAD HQ, Colorado, so without further explanation it's reasonable to suppose they're referring to screens there. There's nothing in the article to say the "injects" affected FAA screens, or NEADS, or even everyone in the command centre. Neither does the article describe the nature of the false information, or even say "radar screens", so the idea of "false blips" is also an invention.

Little support for the claims here, then. What else is there?

2. 11 potential hijackings

There were also reports of other potential hijacked aircraft.

"Jane Garvey, the administrator of the Federal Aviation Administration, responded: “The two aircraft that went in were American flight 11, a 767, and United 175, also a 767. Hijacked.”

“Well, Jane, can you order aircraft down? We’re going to have to clear the airspace around Washington and New York.”

“We may have to do a lot more than that, Dick. I already put a hold on all takeoffs and landings in New York and Washington, but we have reports of 11 aircraft off course or out of communications, maybe hijacked.”"
http://www.thetruthseeker.co.uk/article.asp?id=1648

Some have asked whether these might be connected with the war games. We don't know, however there are other explanations. It's not unknown for planes to stray off course or temporarily lose radio contact, for instance. Normally these might be resolved in a minute or two, but if air traffic controllers are highlighting every problem they face at the current time, then it's not unreasonable to assume they'd flag up potential hijackings. Add human error (ATC and flight crews) and 11 issues, out of over 4,500 planes in or heading towards US airspace, doesn't seem too bad.

A story in USA today emphasises the point:

"At the Federal Aviation Administration's command center in Herndon, Va., Delta Flight 1989 joins a growing list of suspicious jets. Some of their flight numbers will be scrawled on a white dry-erase board throughout the morning. Eventually, the list will grow to 11.

One, a TWA flight, refuses to land in Pittsburgh and wants to fly on toward Washington. Another, a Midwest Express flight, disappears from radar over West Virginia. And three jets over the Atlantic Ocean are sending out distress signals, the Coast Guard reports".
http://www.usatoday.com/news/sept11/2002-08-12-hijacker-daytwo_x.htm

A mix of oddball problems, but without further details there's no evidence to tie them in to the war games.

3. Phantom Flight 11

Others believe false blips were behind confusion over the location of Flight 11.

"There were likely false blips on screen even after 9:16. The Kean Commission's report introduced "phantom flight-11" as being reported by the FAA at 9:25 on 9/11. The FAA reported flight 11 was heading to Washington D.C. at that time when in fact it had already struck the World Trade Center. The Kean Commission's report stated they were "unable to locate the source of the mistaken FAA information."

"Phantom flight-11" was a false blip, but since the war games are classified, specific information on "false blips" and other details can't be reported".
http://www.copvcia.com/free/ww3/011805_simplify_case.shtml

This is stretching the facts a little. All we know is that FAA headquarters reported to Boston Central that Flight 11 was still airborne and heading for Washington. There's no further information as to how this happened. "False blips" are a possible explanation, but then so is human error. 

So despite the claims, there's no direct evidence that "false blips" were placed on FAA radar screens. Even if they were, there's no reason to believe these influenced the military response to the World Trade Centre attacks. The most that can be claimed is they're a possible explanation for the Flight 11 report, but that's all. The evidence for anything further just isn't there.