(Reuters) - The U.S. Department of Justice has subpoenaed a high-profile whistleblower in its criminal investigation into Wells Fargo & Co's WFC.N opening of accounts without customer permission.

A Wells Fargo branch is seen in the Chicago suburb of Evanston, Illinois, U.S. on February 10, 2015. REUTERS/Jim Young/File Photo - RTX2TOT5

U.S. prosecutors in San Francisco have asked Wells Fargo banker Yesenia Guitron, who lost a private lawsuit against the fourth-largest lender, to testify before a grand jury in San Francisco on Tuesday, according to a subpoena dated Dec. 12, which was seen by Reuters.

A Wells Fargo spokesman declined to comment.

Guitron is among at least five Wells Fargo employees who sued the bank or filed complaints with regulators alleging that they were fired after reporting the opening of customer accounts without their permission, according to a Reuters review of lawsuits and complaints to the U.S. Labor Department.

The suits and complaints, filed between 2010 and 2014, raise questions about how early Wells Fargo knew about such allegations and how it handled them.

San Francisco-based Wells Fargo reached a settlement with U.S. regulators and the Los Angeles city attorney in September.

The Justice Department subpoena directs Guitron to bring all documents related to her employment at Wells, including any related to sales practices, discipline “or other form of retaliation taken against you by Wells Fargo or Wells Fargo employees.”

Guitron and her lawyer were not immediately available for comment. A spokesman for the U.S. attorney for the Northern District of California, which issued the subpoena, declined comment.

The bank has acknowledged opening as many as 2 million accounts without customer permission, and it fired 5,300 employees for the behavior. Former staffers have publicly described a culture where they were pushed into hitting unrealistic sales targets and opened the sham accounts to do so.

Wells Fargo chief executive John Stumpf resigned in October after intense questioning before Congress. Beyond the Justice Department probe, Wells Fargo is facing others from various lawmakers and regulators, including a criminal probe by the California attorney general’s office.

Guitron and another personal banker, Judi Klosek, filed complaints with the Occupational Safety and Health Administration, as well as a joint federal lawsuit in 2010, claiming Wells Fargo retaliated against them for blowing the whistle on similar conduct.

Guitron alleged that managers responded by falsifying a paper trail that purported to document her poor performance, forbidding her from taking family medical leave and firing her improperly.

A federal judge ultimately dismissed all of Guitron’s claims, saying Wells Fargo was justified in firing her because she failed to meet sales quotas and refused to meet with management.