New York city officials say 4,964 are dead or missing but none of the lists maintained by others comes near to that. One compiled by the New York Times puts the toll at about 2,950; another, by USA Today, the nation's biggest-selling newspaper, stops at 2,680; and that maintained by the Associated Press news agency goes no further than 2,625.
The American Red Cross, which has received $500m in donations, had expected to be contacted by the families of most victims but has dealt with only 2,563 cases six weeks after the attacks.
"Where are those people?" asked Luis Garcia, the Red Cross official in charge of allocating grants to them.
The huge discrepancy became evident only a day after the New York police department unit responsible for keeping a count said that it was edging closer to a reliable number for the dead and missing, partly by matching body parts to DNA from victims' toothbrushes and hairbrushes.
Anthony De Barros, a computer database editor at USA Today, said: "It has seemed to be odd.
"I don't want to discount the possibility that there were large numbers of people who were just visiting or working in the building that day, or people like carpenters or electricians. But still, even so, it is hard to understand where they are going to come from to reach the numbers the city is reporting."
Numbers have fallen after the initial fear that up to 10,000 people might have been killed, and again after the same names were found to have appeared on more than one list as information poured in from numerous sources. But still the city's total has fallen by only about 500 over the past three weeks.