Sacramento Mayor Kevin Johnson's chief of staff was responsible for reviewing and approving credit card purchases made each month by the former mayoral appointee who allegedly racked up thousands of dollars in personal purchases on her city-issued card, a top city official said Tuesday.

It's unclear whether that oversight ever occurred, said City Manager John Shirey.

City officials were unable to explain Tuesday how long-time mayoral aide Lisa Serna-Mayorga was able to use her city- issued credit card to make personal purchases – among them a trip to Disneyland, groceries and gas, according to sources – without anyone noticing. Serna resigned Thursday, after the questionable charges were discovered.

"I don't know whether (monthly credit card statement) reviews were occurring or if they were occurring consistently," Shirey said. "The only thing I can answer with any certainty is that her direct report was to the mayor's chief of staff."

Shirey has sought the help of both the city Police Department and an outside auditing firm to examine the charges, which have sparked broader concerns about the use of credit cards by city workers. Roughly 300 city employees hold city-issued credit cards, Shirey said, and those accounts will be the focus of an internal audit later this year.

"I've always been concerned by credit card usage by employees," Shirey said. "It doesn't matter what job I've been in. They are easily abused, they are easily mistakenly abused, but you usually catch it right away."

Kunal Merchant was the mayor's chief of staff when Serna-Mayorga made the purchases in question. He recently left that position to lead the mayor's Think Big arena task force.

Mayoral spokesman Joaquin McPeek said, "We are working to get to the bottom of this with as much transparency as possible. As this is a personnel matter, we are unable to comment in detail."

Serna-Mayorga has not returned emailed requests for comment. The voice mailbox for her home phone is full.

Councilwoman Sandy Sheedy said the examination of Serna-Mayorga's credit card use should be conducted with "total and complete transparency."

"The problem is, this makes all of us look really bad," she said.

Johnson said he's confident the incident is an isolated one. "I don't fear this is a trend with city employees," he said.

City Auditor Jorge Oseguera said he already had scheduled an audit of all city credit card use that will go forward as planned in September. Oseguera said he requested permission in February to conduct the citywide audit, before the allegations about Serna-Mayorga surfaced. He said he is following up on a 2008 audit into credit card use within the Department of Utilities.

That examination found questionable purchases made by utilities employees and recommended that officials limit credit card use. In response, utilities officials slashed the number of cards given to employees from 35 to 14, said department spokeswoman Jessica Hess.

The audit will not cover cards used by the mayor or City Council members, to whom the city auditor reports directly. Oseguera said he can't accelerate the audit because he doesn't have staff available until September.

Serna-Mayorga resigned last week after a year-end review of her account revealed she had racked up $9,000 in personal purchases on her city-issued credit card, according to City Hall sources who were not authorized to speak about the matter.

The charges in question came to light at the end of the 2011-12 fiscal year in June. At first, the charges were thought to be limited to $3,000 over the past year, but a deeper analysis found purchases dating back more than a year, the sources said.

Serna-Mayorga reimbursed the city $3,000 in June and another $6,000 last week, the sources said.

The mayor, who has known Serna-Mayorga for years, said she is "someone who feels very badly about a decision that was made."

"Lisa is somebody I love dearly," the mayor said. "She has been a tremendous friend and supporter."

Serna-Mayorga, 39, has been active in St. HOPE Public Schools, a nonprofit founded by Johnson that operates charter schools in his native Oak Park.

She comes from a Sacramento political dynasty. Her father, the late Joe Serna Jr., was a popular mayor. Her brother is Sacramento County Supervisor Phil Serna.

When Johnson was elected, one of his first actions was to appoint Serna-Mayorga as the City Council operations manager, where she oversaw expense reports and other daily tasks for the council and mayoral offices.

In June, Serna-Mayorga was transferred from her council operations position to a post as director of constituent affairs in the mayor's office. It was during that transition that her credit card statements were examined, the sources said.

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