A fifth Toronto police officer has been implicated in the investigation into a controversial drug bust and alleged attempts to mislead a judge that led to criminal charges against four other officers earlier this week, CBC News has learned.

The four officers were charged Thursday with 17 counts of obstructing justice and perjury in relation to a case that ended with an Ontario Superior Court judge dismissing charges against a man accused of possessing heroin.

Numerous sources have told CBC News the fifth officer implicated is being investigated under the Police Services Act but will not be facing criminal charges. But there is no word yet on what charges that officer might face under the Police Services Act.

Charges against the four officers and the investigation of the fifth arise from the arrest of Nguyen Son Tran on January 13, 2014.

Heroin possession charges dismissed

Last September, Justice Edward Morgan dismissed charges against Tran, who was accused on Jan. 13, 2014 of possessing heroin.

The judge concluded the officers concocted a false story about why they stopped the man and searched his car.

Morgan also ruled the officers falsely testified they found loose heroin powder on the dashboard, which led to a search that uncovered 11 more grams of the drug wrapped and hidden behind the car's steering column.

He ruled the drugs seized during the search were not admissible as evidence and dismissed the charges against Tran.

Internal investigators from the Toronto police Professional Standards Unit are probing the conduct of all five officers. They have located a video related to the Tran arrest, sources say.

Video evidence

CBC News has also learned that neither Tran nor his lawyer were told that a dashboard camera video of the incident may exist.

A camera is activated automatically whenever an officer turns on a cruiser's flashing roof lights. A microphone mounted on the officer's shoulder is also capable of recording any verbal exchanges between officers and a suspect.

During Tran's trial, the four officers said they had no recollection of using mobile phones to communicate with each other before or during Tran's arrest.

The internal investigators have since obtained court orders and seized phone records detailing whether, and for how long, the officers talked to each other, sources have told CBC News.

CBC News is not identifying the fifth officer. It's believed the officer is co-operating with investigators.

John Lancaster is a reporter with the #CBCInvestigates unit. He can be reached at john.lancaster@cbc.ca or 416-205-7538.