traditional Special Mention loans category. Classified (substandard, doubtful and loss) loans were $16.0 million at September 30, 2020, $12.5 million at December 31, 2019 and $14.1 million at September 30, 2019. Although we currently do not anticipate COVID-19 to have a material increase to Special Mention or Classified loans, given the industries in which we provide funding, we continue to monitor the situation.
During the three months ended September 30, 2020 there were $104,000 of charge-offs and $62,000 of recoveries, compared to $107,000 of charge-offs and $15,000 of recoveries for the three months ended September 30, 2019.
For the nine months ended September 30, 2020, there were $236,000 of charge-offs and $106,000 of recoveries, compared to $964,000 of charge-offs and $18,000 of recoveries for the nine months ended September 30, 2019.
Comparison of Operating Results for the Three Months Ended September 30, 2020 and 2019
General. Net income for the three months ended September 30, 2020 was $55.0 million, an increase of $34.7 million, or 171%, from net income of $20.3 million for the three months ended September 30, 2019. The increase was primarily due to a $32.7 million, or 100% increase in net interest income that reflected significant growth in mortgage warehouse and multi-family loans, and an increase of $21.2 million, or 255% increase in gain on sale of loans, primarily from higher growth in single-family and multi-family mortgages. Also contributing to the increase in net income was a $4.1 million, or 153%, increase in mortgage warehouse fees and a $767,000 increase in loan servicing fees that reflected a lower negative fair market value adjustment to mortgage servicing rights.
Partially offsetting the increases to net income was a $13.1 million increase in the provision for income taxes due to the 179% increase in pre-tax income, and a $10.9 million increase in noninterest expenses, reflecting higher salaries and employee benefits to support the strong growth in our businesses, as well as increases in loan expenses from higher volume.
Net Interest Income. Net interest income increased $32.7 million, or 100%, to $65.3 million for the three months ended September 30, 2020, compared with the three months ended September 30, 2019. The 100% increase was due to a $3.4 billion increase in our average interest earning assets and a 76 basis point increase in our interest rate spread, to 2.74%, for the three months ended September 30, 2020 from 1.98% for the three months ended September 30, 2019.
Our net interest margin increased 59 basis points, to 2.81%, for the three months ended September 30, 2020 from 2.22% for the three months ended September 30, 2019. The increase in net interest margin reflected lower funding costs that outpaced the lower overall market interest rates on loans.
Interest Income. Interest income increased $16.5 million, or 28%, to $76.3 million for the three months ended September 30, 2020, compared with the three months ended September 30, 2019. This increase was primarily attributable to a $19.1 million increase in interest on loans and loans held for sale from higher volumes and was partially offset by lower overall market interest rates.
The average balance of loans, including loans held for sale, during the three months ended September 30, 2020 increased $3.2 billion, or 68%, to $7.9 billion from $4.7 billion for the three months ended September 30, 2019, while the average yield on loans decreased 83 basis points, to 3.61%, for the three months ended September 30, 2020, compared to 4.44% for the three months ended September 30, 2019. The increase in average balances of loans and loans held for sale was primarily due to significant increases in warehouse funding and multi-family volume. The decrease in the average yield on loans was due to the overall decrease in interest rates in the economy period to period.
The average balance of mortgage loans in process of securitization increased $286.4 million, or 176%, to $449.3 million for the three months ended September 30, 2020, compared to $162.9 million for the three months ended September 30, 2019, while the average yield decreased 58 basis points to 2.88% for the three months ended September 30, 2020.