Social Security Death Index
Did more than 250 people really die on the hijacked 9/11 flights?
It sounds like an absurd question, but it's one that has been taken seriously in the past.
The implication here appears to be either that the passengers are real people who somehow survived 9/11, or perhaps were fictional characters, created as part of the story of the day.
The first wrinkle in this theory comes from the quote itself, which reveals that one of the eleven flight crew, and some of the passengers are included in the index. There is, therefore, a record of the deaths of some of those who were on board the planes. If it's to be claimed that these were faked, then we might ask whether it's credible that the conspirators simply "forgot" to fake the records for the others. But if they're genuine, the only basis for suspicion would be if there are significantly less records in the SSDI than might be expected. And that's an issue on which our article offers no evidence whatsoever.
The reality is that the SSDI is by no means a comprehensive list of everyone who dies in the US. You only need read its own warnings to find out more.
Another page explains other reasons as to "Why You May Not Find Your Ancestors":
It's very clear that this is not an exhaustive or complete database. It may be true that fewer relatives than usual reported the deaths to the SSA, but then these weren't ordinary deaths. Is it really so surprising that the paperwork wasn't completed in all cases? And as Ancestry.com points out, "many funeral directors will notify the SSA as a service to families of the deceased" (http://www.ancestry.com/search/rectype/vital/ssdi/miscon.htm), but if you don't have a funeral then it'll only happen if you remember to do it yourself.