JPMorgan Chase & Co., one of the biggest customers of Bloomberg LP, said it has sent a formal legal request asking the financial data and news company to provide details of what bank information Bloomberg News reporters had been able to see.
JPMorgan's statement comes after Bloomberg acknowledged late last week that its reporters had limited access to data about clients' terminal usage, such as when a customer logs in, contacts the help desk or delves into the system for information about assets, such as equities or bonds.
The largest U.S. bank is seeking logs for five years of what precisely Bloomberg journalists accessed concerning the use of terminals by JPMorgan employees, a bank official said. Bloomberg has about 2,400 journalists worldwide.
JPMorgan said it is also seeking "confirmation" of controls that Bloomberg has put in place to stop future breaches.
The bank declined to provide a copy of what it described as a formal request from its legal department.
A Bloomberg spokesman declined to comment.
Bloomberg, which competes with Thomson Reuters Corp., the parent of Reuters News, has said it restricted reporters' access to that information last month after another client, Goldman Sachs Group Inc, complained.
Goldman flagged the matter to Bloomberg after the bank found that journalists had access to more information than it had realized, arguing the information was sensitive and should not be seen by reporters.
On Wednesday, Goldman President Gary Cohn told CNBC that the bank does not have a major concern about the issue.
The world's biggest central banks, including the U.S. Federal Reserve and the European Central Bank, are also Bloomberg customers and said they are asking the media company what journalists could learn about their terminal usage as well.
JPMorgan public relations staffers fumed last year that Bloomberg reporters had checked when bank employees had last used terminals to discern who might have been dismissed during the bank's investigation of its $6.2 billion London Whale trading loss.
On Monday Matthew Winkler, editor-in-chief of Bloomberg News, apologized for allowing journalists "limited" access to sensitive data about how clients used Bloomberg terminals, saying it was "inexcusable", but that important customer data had always been protected.
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