The serious delinquency rate on mortgages backed by Fannie Mae dropped to 4%, the lowest level since June 2009.

The rate fell every month since the 5.59% peak in February 2009 except for July when it went unchanged. Fannie said the rate fell 3 basis points from August.

Fannie purchases from lenders and servicers totaled $55.3 billion in September, up 24% from the previous month but down 33% from the same month last year. It is the highest volume since the $57 billion reported in March.

Fannie's gross mortgage portfolio dropped to $722.1 billion in September, down 10% from one year ago.

Under the conservatorship agreements established in September 2008, the Treasury Department capped this portfolio at $900 billion at the end of 2009 and scheduled it to be wound down by 10% each year, reaching $250 billion by 2018.

Write to Jon Prior.

Follow him on Twitter @JonAPrior.