Berkshire Hathaway (BRK.A 0.61%) (BRK.B 0.79%) just added shares of Bank of America (BAC 0.09%) to its already massive stake for the third time in the past two weeks.

According to regulatory filings, CEO Warren Buffett and his team purchased another 21.2 million shares of the bank between Tuesday and Thursday of this week for a total cost of $522 million, or an average price of $24.65 per share.

Warren Buffett smiling and talking to reporters.

Warren Buffett. Image source: The Motley Fool.

This comes on the heels of two other significant purchases of the banking giant's shares. Last week, Berkshire spent more than $800 million to buy 34 million shares of Bank of America, and quickly followed that by spending an additional $400 million on the stock. In all, Buffett has spend about $1.7 billion to add to the Bank of America stake in the past two weeks alone.

After the recent additions, Berkshire now owns more than 1 billion shares of the bank with a total market value of $25.3 billion. The 11.8% stake is the second-largest position in Berkshire's portfolio and is the largest of Berkshire's many bank stock investments by a wide margin.

Buffett could be adding to other bank positions as well

It's worth pointing out that the only reason we know about these recent purchases is because Berkshire's ownership stake is greater than 10%, which comes with additional regulatory scrutiny. We won't find out about other stocks Buffett has recently bought or sold until Berkshire releases its 13-F filings, which are due out in mid-August for the second quarter and mid-November for the third.

In short, Buffett could be adding to Berkshire's other bank stock positions as well, such as JPMorgan Chase (JPM 0.51%), of which Berkshire owns less than 2%, or PNC Financial (PNC -0.18%), of which Berkshire owns a 2.2% stake. Historically, Buffett has added to bank stocks in groups, so I wouldn't be surprised to see other bank stakes increase as well over the coming quarters.