Fairfax County Public Schools discovered the administration mistakenly hired seven employees who are convicted felons, including a high school teacher who was once part of a trafficking ring that conspired to import more than a kilogram of heroin.

School officials uncovered the mistake during a comprehensive search of hiring records that began in 2012 after a Madison High School special education teacher inquired whether her 1992 conviction for drug trafficking would stall her promotion to an administrative position.

That employee, Deilia Butler, joined Fairfax schools in 2006. She had stated truthfully that she was a convicted felon on her application. So did the other six employees, school officials said.

Virginia law prohibits school divisions from hiring felons.

“Human error played a part in these hirings and we deeply regret this mistake,” Superintendent Karen Garza said in a statement.

None of the seven employees remain in their positions, school officials said. Four, including Butler, were placed on administrative leave after school officials uncovered the error. The three others left the school system. Garza, who took over as superintendent in July, did not specify how the convictions initially were overlooked.

School officials have declined to name the other employees who were convicted felons or disclose their positions. Officials also have refused to detail the nature of the employees’ crimes or discuss whether they had discipline problems or other issues while working in the school system.

The hirings all occurred before 2009, when Fairfax schools switched to an online application, school officials said. The new system automatically disqualifies potential employees who disclose felonies on their application.

All applicants go through background searches that are supposed to include checking FBI, police and child abuse databases, school officials said. None of the seven employees committed crimes against children, Garza said.

The issue became public when Butler, who administration officials said had a respected history with Madison High School, was named in a lawsuit filed by the school system in Fairfax County Circuit Court.

The administration asked the court to affirm that the school system has the authority to fire Butler, who has been on paid administrative leave since February 2013. Garza said in a statement that she is “confident” that the court will allow the administration to move forward with Butler’s dismissal.

Butler declined to comment Friday and said she would talk to her attorney, who was not identified in court records. But the lawsuit says Butler is fighting the schools’ efforts to fire her, claiming Virginia law did not bar the school system from hiring her.

The three other employees on administrative leave also may be fired, pending the outcome of the court case, school officials said.

Kimberly Adams, president of the Fairfax Education Association, a teacher’s union that represents more than 5,000 members, said the administration had not properly handled Butler’s case.

“No employee should be denied their job or promotion status based on previously disclosed charges at the beginning of their employment, especially if those disclosures were acceptable at the time of hiring and the employee has worked successfully throughout their career,” Adams said.

Garza on Monday noted that the revelations involving the felons’ employment began before her tenure. “In my estimation, once it came to the administration there was immediate action taken,” she said. “We have the right systems in place to make sure this never happens again.”

Eleven of the 12 school board members declined an in-person request for comment at a board meeting Monday evening. The 12th member, Vice Chairman Tammy Derenak Kaufax, who was absent from the meeting, did not return a request for comment.

Court records show that Butler and a handful of others were charged in a 34-count federal indictment of conspiring to import more than a kilogram of heroin into the United States. She served 42 months in prison before she earned her teacher’s license and later joined Fairfax schools, the lawsuit says.

According to court filings in the civil case, Butler apparently asked a school official whether her criminal record would prevent her from advancing within the school system. Alerted to the situation, school human resources officials began combing records for others who may have had felony convictions.

A search of 19,000 hires between 1996 and 2009 found the six additional employees with felony convictions, school officials said. By spring 2013, they all had been removed from their positions.

In a March 29, 2013, letter, Butler asked a federal judge in Ohio to expunge her conviction for a “one-time irresponsible act and behavior that I allowed myself to get caught up in.”

Jennifer Jenkins contributed to this report.