TAMPA — Tampa police detectives eavesdropping on a jail inmate's phone calls heard a voice they did not expect:

Their own agency's Sgt. La Joyce Houston talking to an inmate named Rita Girven, the biological mother of a girl Houston had adopted years ago.

The two women conspired to let Houston use Girven's food stamps, doled out on an electronic benefits transfer, or EBT, card, police say. On Sept. 10, Houston used it to buy $365 in groceries at a Gibsonton Walmart.

On Thursday, police fired, then arrested 47-year-old Houston on two charges of food stamp fraud and one charge of grand theft.

The 16-year agency veteran made $91,400 a year. By 3 p.m. Thursday, she was sitting in the Orient Road Jail, held in lieu of $6,000 bail.

If convicted, she might forfeit her pension, which is about $46,000 a year, officials said.

Houston is married to Tampa police detective Eric Houston, a respected homicide investigator, who went to work Thursday as usual. Police Chief Jane Castor said the agency does not believe he was involved with — or knew about — any fraud. Nonetheless, the agency plans to conduct an "exhaustive" internal investigation, she said.

La Joyce Houston's is the department's third public firing in two weeks. On Sept. 27, the agency fired and arrested a detective accused of stealing from the evidence room. They also fired Sgt. Ray Fernandez, the DUI supervisor involved in what appears to have been the setup of a local lawyer. Fernandez has not been charged.

Though the trio's dismissals happened in quick succession, Castor said officers like them make up a small part of the agency, which employs just under 1,000 sworn officers.

"It's very rare for one of our officers to violate policies, much less the law," Castor said.

Detectives first learned of Houston's phone conversations on Aug. 8, as they secretly listened to Girven's calls as part of a separate investigation, Castor said. Girven was booked into jail on Aug. 8 on charges of petty theft and violation of probation. She has a long criminal record with 49 felony charges and 14 convictions, police say. Most are related to theft and fraud.

Over nine weeks, Houston and Girven spoke at least 35 times, and during one of the calls, Girven told the sergeant where she could find Girven's EBT card, police say. In addition to using it at Walmart, police say Houston also gave it to one of her family members to use in exchange for 50 cents for every dollar spent.

Houston put the proceeds into Girven's jail canteen account, police say.

Girven was charged with welfare fraud in connection with this case, police say.

Another problem: Girven's EBT card was in the name of her 12-year-old son, police say. That is not allowed because Girven neither had custody of the boy nor provided him any financial support.

Houston was told of her firing Thursday morning at the office of her attorney, Daniel J. Fernandez. Reached in the afternoon, Fernandez said he was not ready to talk case specifics — only about his client in general.

Houston supervised night-shift patrol officers in east Tampa, Ybor City and the Port of Tampa and had no previous disciplinary issues in her file, police say. While moving up the ranks at the Tampa Police Department, she got a bachelor's degree in sociology and a master's in education, her attorney said.

She married detective Eric Houston in 2007, and in 2009 the two legally adopted the young girl that Houston had cared for since infancy. The girl is now 9.

On Thursday, no one had an answer for why. Why would Houston steal several hundred dollars and risk her good-paying job and pension?

Chief Castor said Houston gave no statement and remained stoic as she was told of her firing.

Times news researcher John Martin contributed to this report. Jessica Vander Velde can be reached at jvandervelde@tampabay.com or (813) 226-3433.