Commercial loans remained the driving force behind bank failures in September, according to commercial real estate data analytics firm Trepp.

Six banks failed last month — down from seven in August — bringing the number of bank failures to 74 in 2011. Nearly 300 financial institutions failed in 2009 and 2010 combined.

In evaluating the banks shuttered in September, Trepp discovered CRE exposure contributed significantly to the banks' insolvency.

CRE loans accounted for $365 million, or 82%, of the total nonperforming loans at failed banks in September. Within that group, commercial mortgages fared the worst making up $199 million, or 45%, of the total nonperforming loan pool, while construction and land loans made up $166 million, or 37% of the pool.

The residential real estate loan category had $61 million in nonperforming loans, representing 14% of the total nonperforming loan balance at the failed banks, two of which were in Georgia with other failures in Florida, Virginia, California and Texas last month.

Write to Kerri Panchuk.