A former Billerica Police officer is accused of the aggravated rape of a 15-year-old girl while he worked for the department.

A Middlesex Superior Court grand jury indicted Alexander Allen, 35, on charges of the aggravated rape of a child with a 10-year age difference and possession of child pornography. He is scheduled to be arraigned on the charges at the Woburn courthouse on Oct. 25 at 10 a.m.

“The allegations against Allen, if proven, are deeply disturbing and represent a betrayal of the oath sworn by police officers to protect the innocent and serve the public with integrity,” Billerica Police Chief Roy Frost said in a statement. “There is no room in the Billerica Police Department for the abhorrent behavior alleged in this case.”

Frost’s statement added that the department first became aware of the allegations on Dec. 3 of last year. The Middlesex District Attorney’s office in a statement said that he told another officer he was having an inappropriate relationship with a then-18-year-old girl.

But investigations by both the police department and the DA’s office found that the relationship went back further, to when the girl was underage.

Prosecutors say that Allen first met the girl when she was 14 years old and then “grew close to her.” Then in March 2021, while he worked as a Billerica Police officer, he picked her up from an unspecified appointment, drove her back to her home where they were alone and “engaged in sexual relations with the victim.” He also allegedly solicited a naked photo of the girl before her 18th birthday.

Frost said that he placed Allen on administrative leave on Dec. 5, advised the Massachusetts Peace Officers Standards and Training (POST) Commission that Allen’s police officer certification be temporarily suspended and hired Discrimination and Harassment Solutions LLC to conduct an independent investigation. The department then informed the DA’s office of the matter.

The internal investigation was suspended on Dec. 15 in favor of a DA criminal investigation and on Dec. 18 Allen resigned.

Another police officer, who has not been identified, was placed on paid administrative leave on Tuesday based on materials contained in Allen’s indictment, according to the police department.