A senior associate at a lobbying firm tapped to help bring a controversialWal-Martgrocery store to Chinatown abruptly left the firm Thursday after it was revealed that she had infiltrated a news conference held by a group critical ofWal-Martand posed as a reporter.

Stephanie Harnett had been an employee of Mercury Public Affairs, which received $60,000 from Wal-Mart to lobby L.A. City Hall between January and March, according to Ethics Commission records. The proposed project has provoked opposition from labor groups, which complain about unfair working conditions at the retailer.

On June 6, one of those groups held a news conference to discuss labor conditions at warehouses that are retained by Wal-Mart. According to Elizabeth Brennan, a spokeswoman for Warehouse Workers United, Harnett showed up at the meeting and filled out a media sign-in sheet, saying she was a USC student named "Zoe Mitchell."

According to Brennan, Harnett conducted a 20-minute interview with a warehouse worker, using an audio recorder, without revealing she worked for Wal-Mart. "She told him she was a journalism student at USC and that she was a storyteller from the heart," Brennan said.

Brennan said she saw Harnett again Wednesday at a news conference focused on an upcoming demonstration opposing Wal-Mart's Chinatown store, which will include a supermarket and pharmacy. At that event, an activist pointed out that she was not a USC student but rather an employee of Mercury.

Harnett's actions, first reported by the website Gawker, drew swift condemnations from her supervisors at Mercury.

"The action taken by Ms. Harnett was in no way approved, authorized, or directed by Wal-Mart or Mercury," Becky Warren, managing director for Mercury, said in a statement.

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-- David Zahniser and Kate Linthicum, Los Angeles Times