State and federal regulations also require companies to notify customers when their information has been improperly released, as Wells Fargo may now do. And some of the accounts in Mr. Sinderbrand’s database are listed as having a foreign owner, which would potentially trigger a separate set of overseas regulations, such as Europe’s stricter privacy statutes.

“There are thousands of documents in here that the public should never see,” Mr. Sinderbrand said, noting that a less scrupulous recipient of such data could have easily posted it online.

Reached on Friday, a day after Ms. Turiano was made aware of the issue, a spokeswoman for Wells Fargo Advisors, Emily Acquisto, released the following statement: “Wells Fargo takes the security and privacy of our customers’ information seriously. We are investigating this matter and will take the proper steps based on the outcome of our investigation.”

Ms. Turiano and a spokeswoman for her firm did not respond to requests for comment.

The New York Times was shown large portions of the data and confirmed that it included what appeared to be clients’ names, unredacted Taxpayer Identification Numbers, assets under management, portfolio performance, mortgage information and details on 529 education savings plans.

One file, for example, contained details on the holdings of a well-known hedge fund billionaire who had at least $23 million invested through Wells Fargo Advisors.

The files also include extensive information on Wells Fargo’s financial advisers employed by the bank, their performance, their compensation and their client lists. One typical record showed the full roster of one adviser’s client book and his commissions for the past year, totaling $1.5 million.

Based on the fairly narrow subpoena that his lawyer submitted — it sought communications about Mr. Sinderbrand’s employment and compensation — there was no reason for the bank to turn over such information, especially without any redactions, Mr. Sinderbrand said.

“This is a public policy issue,” he said. “They have to find out what happened and how it happened. Did it happen before, and could it happen again?”