(CNN) -- All her life, Carmen Figueroa believed she was born in the United States. It was what her mother had always told her.
But a State Department investigation found that she was born in Sinaloa, Mexico, and brought into the country illegally at a young age, CNN affiliate KVOA reported. The discovery brought an abrupt end to Figueroa's 10-year career with the Arizona state police.
Figueroa, 42, was forced to resign or be fired. Under Arizona law, an undocumented immigrant cannot be a peace officer.
Her record as an officer included praise and promotions.
The case is a reminder of how the country's immigration laws can turn lives upside down. It raises questions about how potential public employees are screened, and how easily facts about something as basic as a birthplace can be obscured.
"She was great, well-liked. The whole thing is a sad story," said Bart Graves, a spokesman for the Arizona Department of Public Safety.
Figueroa had joined the force as a highway patrol officer 10 years ago and became a detective in 2010.
The detective likely would have continued to serve if not for the State Department investigation.
Figueroa's brother had applied for a passport while he was in the Air Force, Graves said, and during that process the State Department flagged him about his citizenship.
"When she was informed by the State Department that she and her brother were not U.S. citizens, that was the first she'd heard of it," Graves said. "Her mother had told her she was born in this country."
Figueroa learned of her immigration status in June, but she didn't inform her agency, Graves said.
"She knew in June about her status and by law was required to report it immediately," he said. "She waited for us to find out about it in August."
Once it found out, the Department of Public Safety placed Figueroa on paid administrative leave until Monday, when she resigned.
The State Department conducted a criminal investigation and in October decided that it will not file any charges against Figueroa, Graves said.
Arizona continues to work with federal officials to determine if any state criminal charges will be filed, Graves said.
Figueroa could not be reached for comment despite repeated calls.
In Arizona, a state known for its law cracking down on illegal immigration, the case has sparked debate.
It's unclear what's next for Figueroa, KVOA reported.
"A situation like this where someone has been in the U.S. for 30 or 40 years believing that they're a U.S. citizen is pretty rare," immigration lawyer Maurice Goldman told the CNN affiliate.
But one immigrant rights activist told CNN en Español such cases aren't as uncommon as you might think.
Dulce Matuz said she knows people who've found out they were undocumented when they went to get driver's licenses. Many of them grew up during a different climate surrounding immigration, she said.
"This is a topic that wasn't talked about," Matuz said. "It was taboo. ... Now we are seeing that children are realizing at a very young age the immigration status of their parents and themselves."
CNN's Stan Wilson and journalist Valeria Fernandez contributed to this report.