With the data trail cold, one working theory both inside and outside the bank is that internal documents in Mr. Assange’s possession, if any, probably came from the mountains of material turned over to the Securities and Exchange Commission, Congressional investigators and the New York attorney general’s office during separate investigations in 2009 and 2010 into the bank’s acquisition of Merrill Lynch.

As it happens, Mr. Assange’s first mention of the Bank of America hard drive, in October 2009, coincided with hearings by the House Committee on Oversight and Government Reform into the Merrill merger, and with wide-ranging requests for information by the committee.

The bank’s investigative team is trying to reconstruct the handover of materials to public agencies for a variety of inquiries, in pursuit of previously undisclosed documents that could embarrass the company, bank officials said.

In addition to the Merrill documents, the team is reviewing material on Bank of America’s disastrous acquisition in 2008 of Countrywide Financial, the subprime mortgage specialist, the officials said. The criticism of Bank of America’s foreclosure procedures centers mostly on loans it acquired in the Countrywide deal, and one possibility is that the documents could show unscrupulous or fraudulent lending practices by Countrywide.