While federal investigators were uncovering the largest government theft in the city's history inside the District's tax office, another employee apparently was running her own embezzlement scheme down the hallway.

According to charging documents made public Tuesday, Mary Ayers-Zander began stealing hundreds of thousands of dollars from the Office of Tax and Revenue in February 2007, about the same time that D.C. tax office manager Harriette Walters' 20-year, $50 million theft scheme was unraveling.

The alleged $400,000-plus theft by Ayers-Zander, a tax examiner assistant, continued until January of this year despite assurances from D.C. Chief Financial Officer Natwar Gandhi that he had changed the culture of the office and had established stricter internal controls to prevent large-scale theft by workers in the office.

David Umansky, a spokesman for the CFO's office, said the measures taken by the office after the Walters case helped detect the Ayers-Zander theft.

"We were the ones who caught her," Umansky said.

The new safeguards closed off a loophole in the system that prevented more thefts for about a year-and-a-half, he said. In January, a tax office official noticed an unusual pattern of refunds being paid and the office notified the FBI, the city's inspector general and auditor.

The FBI determined that no other OTR employees were involved, Umansky said.

Ayers-Zander was charged Monday by information, which typically means that a plea deal is in the works.

Ayers, who no longer works for the city, faces a maximum of 20 years if convicted. A plea hearing has not been scheduled.

Her attorney, Peter Fayne, said he had no comment.

According to charging documents, Ayers-Zander ran several scams to steal $414,800.

She fraudulently credited the taxpayer accounts of several people and had $365,000 wired to her own bank personal bank accounts in 48 separate transfers, documents said.

Ayers-Zander also fraudulently credited the accounts of other individuals and wired $46,000 in fraudulent refunds to those individuals by check or electronic funds transfer, according to the charging documents.

She also filed fraudulent income tax returns for individuals and had refunds wired to an individual's bank account in California, documents allege.