Officials with New York Mellon said that when they learned of the breach, they notified the Office of the Comptroller of the Currency, the New York State Banking Department, the Federal Reserve Board, the Securities and Exchange Commission and law enforcement agencies.
However, Blumenthal said his office is continuing its investigation into why it took New York Mellon more than eight weeks to alert the affected institutions of the breach, which he described as "massive, inexplicable and inexcusable."
Some Connecticut residents, including Bruce Sylvester of Hamden, say they are having trouble obtaining information from New York Mellon as to whether their personal information is included on the missing tape.
"My wife and I have the same account," Sylvester said Friday. "We called up Mellon the same day and got two different answers as to whether our information was on the tape."
This week, New York Mellon also revealed a second security breach in which a computer data storage tape containing the images of scanned checks and other documents was lost in late April as it was being transported from Philadelphia to Pittsburgh.
"We are only now getting our arms around this much smaller incident," Bank of New York Mellon spokesman Ron Sommer said Friday. "I don't know if it involves Connecticut residents."
The second breach affects 47 institutional clients, company officials said.
Last week, several customers of People's United Bank filed suit over the data breach. The lawsuit, which seeks class-action status, was filed in Superior Court in New Haven against both People's United and The Bank of New York Mellon.
Mike Stratton, a New Haven attorney representing those suing the banks, described New York Mellon's latest offer of two years of credit protection as inadequate.
"The Bank of New York Mellon is acting with an incredible amount of gall," Stratton said Friday. "First, they recklessly lose the personal information of 4 million people, they then hide it from public view."
Stratton said Mellon must provide "at least seven years of credit monitoring and credit insurance."
The 25 companies identified Friday are: Bank of New York Mellon Corp., People's United Financial Inc., John Hancock Financial Services Inc. , The Walt Disney Co., TD Bank Financial Group, Hudson United Bancorp, United Parcel Service Inc., Wachovia Corp., MetLife Inc., Hudson City Bancorp, Eastman Kodak Co., Burlington Resources, Providian Financial, Penn Fed Financial, ADESA Inc., Alcatel-Lucent, Odyssey America Reinsurance Corp., Seacoast Financials Services Corp., Viewpoint Bank, Diamond Shamrock, Sound Federal Bancorp, Big Lots Inc., Guidant Corp., New York Community Bancorp and ACE Ltd.