The Redbook Average fell 3.2 percent during the first two retail weeks of September, a bigger fall than last week's 0.7 percent loss and compared with a target of flat sales, Redbook said.
The Bank of Tokyo-Mitsubishi and UBS Warburg's retail chain store sales index also fell, slipping 1.4 percent lower for the week ending September 15.
``Clearly the week took a hit on sales and took a hit on traffic. It probably means the month is going to come in a lot weaker than previously anticipated,'' said Mike Niemira, senior economist at Bank of Tokyo-Mitsubishi.
On a year-over-year basis, Redbook's index fell 3.5 percent, compared with a 1.7 percent rise one week earlier. BTM's index grew 2.2 percent on a year-over-year basis, a significant deceleration from last week's 3.3 percent growth.
Year-over-year growth in BTM's index has now been slowing for four consecutive weeks.
``Most retailers were impacted by Tuesday's tragic events and closed on that day,'' Redbook said.
BTM said it expects sales to increase a meager 1.5 to 2.0 percent in September compared with the same month in 2000. In its previous report, BTM had forecast a 3 percent rate of growth for September.
A large drop in customer traffic at chain stores compared with the same time last year -- the 20th consecutive weekly decline -- was directly linked to Tuesday's attacks, BTM said.
``Although customer traffic recovered on Wednesday, sales did not,'' the BTM report said.
The Redbook Retail Sales Average is a sales-weighted average of annual growth in same-store sales at discount, department and chain stores that report their results on a weekly basis.
The BTM/UBSW Weekly Chain Store Sales Snapshot is compiled from seven major discount, department and chain stores across the country which report their weekly results.