A detective for Arizona's state police force has resigned following the discovery that she was living in the country illegally after being brought to the U.S. from Mexico by her family at a young age, the agency said Wednesday.

The former detective, Carmen Figueroa, apparently was told by her family that she was born in the U.S., though she was actually born in Sinaloa, Mexico, said Department of Public Safety spokesman Bart Graves.

Figueroa's status was discovered when the State Department processed a passport application submitted by her brother, who is serving in the U.S. military, Graves said.

Figueroa, 42, resigned Monday but would have been fired if she hadn't stepped down, Graves said. A criminal investigation continues, he said.

Arizona law requires sworn police officers to be U.S. citizens, and Graves said the state police agency would have terminated her for fraud and misrepresentation for "not meeting the qualifications."

Her status was first reported by Tucson television station KVOA.

Figueroa worked in southern Arizona and was with the department for 10 years, most of that time as a Tucson-based Highway Patrol officer. She became a criminal investigations detective in 2010.

"She told us she was always under the impression through her mother that she was born in this country, and she did not really find out until this summer ... that she was an illegal alien through, I believe, a confrontation with her mother," Graves told The Associated Press.

Figueroa "had an exemplary record" and the circumstances of her departure don't put any investigations or prosecutions at risk, Graves said.

Efforts to reach Figueroa for comment were not immediately successful. Calls to a Tucson-area phone listed under her name were not answered, and Graves said he did not know whether she has an attorney.

He said the Department of Public Safety was informed of Figueroa's status Aug. 22, and she was placed on administrative leave with pay Sept. 4.

"It was pretty clear as it went up the chain (of command) that she would be terminated," he said.

According to Graves, Figueroa previously worked for the federal Bureau of Prisons as a secretary and for the Pima County court system as a secretary and an officer. When Figueroa was hired to the state police force, she submitted a birth certificate from Texas and a driver's license and high school diploma from California, Graves said.

There was nothing suspicious at the time about the documents, but Graves declined to discuss whether authorities now question the legitimacy of any of the papers.

However, he said, "we are absolutely having in-depth discussions right now to prevent this from happening again."