In California, it was people with identification cards issued by Mexican consulates. The absence of a Social Security number made it simpler for Wells Fargo employees to open fraudulent accounts in those customers’ names. Wells Fargo is one of the few major banks to permit accounts to be opened without Social Security numbers.

And in Illinois, one former teller described watching in frustration as older customers fell prey.

“We had customers of all ages, but the elderly ones would at times be targeted, because they don’t ask many questions about fees and such,” Brandi Baker, who worked at a branch in Galesburg, Ill., said in an interview.

When Mr. Stumpf testified before members of Congress — once in the Senate and once in the House — he was pressed hard on whether any demographic group had been disproportionately affected. He said he was not sure.

Wells Fargo does not collect information on its customers’ ethnicity, Mr. Stumpf said. Of the two million potentially unauthorized accounts the bank uncovered in its internal review, the affected customers “skewed to younger people, not older people,” he told the House Financial Services Committee.

In the Los Angeles area, for instance, college campuses were considered prime spots for employees seeking to rack up new accounts because younger customers had a tendency to trust a banker’s advice.

Athena McDaniel-Watkins, a former teller who worked in and around Los Angeles, said a banker she worked with would take stacks of forms with him on campus visits and encourage busy students to sign the blank papers — he would fill them out later, he told the students.