A retired Edmonton drug detective charged with extortion and obstruction of justice turned to the "dark side" when he became a private investigator, an Edmonton court heard Monday.

Ross Barros was with the Edmonton police for a quarter of a century and when he retired in 2001, he hung up his shingle as a private investigator.

A defence lawyer hired Barros two years ago on a drug case and asked him to find out the identity of a confidential police source.

Det. Kevin Brezinski, a member of the police organized crime unit who was the lead investigator on the case Barros was investigating, testified Monday that Barros contacted him in 2005.

Barros named the person he thought was the confidential informant, then talked about all the cases that were dropped when he was a drug officer because the informant's identity was leaked, court heard.

Brezinski said he was worried for the safety of his source at that point.

"You were a good cop," he recalled telling Barros. "Why would you go to the dark side and try and ruin someone's life?"

If convicted, Barros faces a maximum penalty of life in prison for extortion and obstruction of justice. The trial is expected to last two weeks.