OMAHA, Neb. — Warren Buffett's company has purchased another $400 million of Bank of America stock less than a week after buying roughly $800 million of the bank's stock.

Berkshire Hathaway Inc. said Monday it held 998 million Bank of America shares after the latest purchases, which represents roughly 11.5% of the bank's stock. Berkshire bought 16.4 million shares of Bank of America stock between Thursday and Monday.

Last week, Berkshire reported buying 33.9 million shares of Bank of America stock worth about $818 million.

Even before this month's purchases, Bank of America Corp. was already the second-largest investment in Berkshire's portfolio behind only its Apple stake.

Investors follow what Berkshire buys and sells closely because of Buffett's successful track record. The Securities and Exchange Commission filing doesn't make clear who made the bank investment, but Buffett generally handles the biggest investments in Berkshire's portfolio, such as Coca-Cola, Apple and Wells Fargo. He has said that investments of less than $1 billion are likely to be the work of Berkshire's two other investment managers.

Besides investments, Berkshire owns a variety of more than 90 companies including insurers such as Geico, BNSF railroad and several large utilities.