Earlier, in a memo to staff on Friday, Bloomberg chief executive Daniel Doctoroff had said that while the company had "long made limited customer relationship data available to our journalists... we realise this was a mistake".
Goldman Sachs became worried after a reporter investigating the possible departure of a Goldman employee told the securities firm that the person had not logged into a Bloomberg terminal for weeks.
JP Morgan reportedly was also targeted, according to the Associated Press.
A person close to the matter was quoted by AP as saying that a number of Bloomberg reporters had used the data to try to score scoops, including finding out whether disciplinary action had been taken against Bruno Iksil, a JP Morgan trader nicknamed the London Whale, who was accused of a $6bn trading loss last year.
JP Morgan reportedly complained to Bloomberg, but the media firm said it was not aware of a formal complaint.
Separately, a Bloomberg spokesperson said the Federal Reserve had also contacted Bloomberg to see whether journalists monitored terminal activity by Fed Chairman Ben Bernanke and former Treasury Secretary Timothy Geithner.
The European Central Bank (ECB) said it had also been in touch with Bloomberg.
"The ECB takes the protection of confidentiality in the usage of data products by ECB management and staff very seriously," an ECB spokesman told Reuters. "Our experts are in close contact with Bloomberg."
Reporters could see whether clients, including many stock and bond traders, looked up news stories or data, although not the content of the information.
Reporters could also see if subscribers were using "message" or "chat" functions to send messages to each other over the terminals, but not their content.
In his staff memo, Mr Doctoroff said reporters did not have access to "trading, portfolio, monitor, blotter or other related systems or our clients' messages".
Reporters were mostly getting contact information for subscribers, such as telephone numbers and email addresses, he said.
No journalists have been fired over the matter, but senior executive Steve Ross has been appointed to oversee client data compliance to review Bloomberg's policies, the company said.
The heart of Bloomberg's business is providing financial data. That is where it makes its money.
The terminals you'll find on any trader's desk account for the bulk of its revenue and it has 315,000 subscribers around the world.
So it won't want its expanding news division to threaten the main source of its profits. And that is the risk here.
At least it appears Bloomberg's management think so judging by their very public apology. The company has admitted it made a mistake and has created a new position to oversee data security issues.
Management want to make sure that the line between its news and business operations isn't crossed again.
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