NEW YORK (CNN) - Ameritrade said Tuesday account information may have been lost for up to 200,000 customers when a package containing tapes with back-up information on customer accounts went missing.

A spokeswoman for the online broker said it was told in February that a package holding four data cassettes containing current and former Ameritrade (Research) account holders' information from the years 2000 through 2003 was misplaced by a shipping company Ameritrade uses. It declined to name the company.

Three of the four tapes were recovered at the shippers' Maryland facility, said the spokeswoman, Donna Kush. The one tape that remains missing contains information on as many as 200,000 current and former customers, she said. Ameritrade has about 3.7 million customers.

Kush says Ameritrade has reviewed the customer information that would be on the missing back-up tape and has decided that only 175,000 of those customers needed to be notified, in accordance with industry standards. The company began sending letters to those customers last week.

"We feel like we acted in a timely fashion, " Kush said. "This was not an Ameritrade Systems issue or a compromise of our technology. This was related to a third party vendor."

She said Ameritrade has every reason to believe that the missing fourth tape has either been destroyed or is still somewhere in the shipper's facility. In addition, she said, the missing back-up tape contained compressed data that would require very advanced computer systems to access.

To date, none of Ameritrade's customers have reported that their personal information has been compromised, Kush said, noting this is the first time Ameritrade has lost customer account information.

Ameritrade, a subsidiary of the Ameritrade Holding Group, was formed in 1983. The parent company's stock edged higher on Nasdaq Tuesday.

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