Ontario's police watchdog is investigating an allegation of sexual assault against an Ottawa Police Services officer that is said to have happened about six years ago.

The Ontario Special Investigations Unit said they invoked their mandate in January when they were first notified. They confirmed the investigation after a story was published in the Ottawa Citizen Sunday night.

"To protect the identities and privacy of the complainant and the subject officer, the SIU does not release information in cases involving allegations of sexual assault, unless there is an appeal for witnesses or information, or the director causes a charge to be laid," said spokesperson Monica Hudon.

The SIU has identified five officers connected to the investigation: one subject officer and four witness officers. Subject officers are not compelled to present themselves for an interview with the SIU and they do not have to submit their notes.

Hudon said they will not be releasing or confirming any further details "in order to protect the identity and privacy of the complainant ‎and others involved."

The SIU is an arm's-length agency that investigates reports involving police where there has been death, serious injury or allegations of sexual assault.