Americans repaid their credit card balances at the slowest monthly rate in four years in September, a sign of deterioration in their credit worthiness, bond rating service Moody's Investors Service said on Wednesday. 

The rate at which cardholders repay their balances fell for a second straight month in September, to 13.79 percent vs 14.25 percent a year earlier, Moody's said. 

Moody's did not tag the economic fallout from the Sept. 11 attacks in New York City and Washington, D.C., as the sole factor that led to the year-to-year drop in balance repayment. The agency reasoned that the fall could have resulted from lower card usage or fewer collection days in September. 

"The number of payment processing days in a month can create month-to-month volatility in the reported payment rate,'' William Black, a Moody's senior analyst, said in a statement. 

Moody's pointed to other signals that U.S. consumers' credit quality has worsened since the Sept. 11 attacks. They included credit card issuers writing off more bad loans and increased lateness for cardholders making their payments. 

The September charge-off rate, or the amount of bad loans written off as uncollectable, rose to 6.18 percent from 5.06 percent a year earlier. The rise in September marked the eighth consecutive month in which the charge-off rate rose from its year-ago level, Moody's said. 

The September delinquency rate on loan balances 30 or more days past due rose for the 10th straight month, to 5.04 percent from 4.69 percent a year earlier. 

"Therefore, a rise in delinquency rate will likely translate to higher losses down the road unless collection efforts on such balances improve,'' said Christophe Germain, a Moody's analyst.