ATLANTA -- A former Atlanta police officer on trial for a botched drug raid that led to the death of a 92-year-old woman said Wednesday he went along with a cover-up because he felt threatened by his fellow officers.

Arthur Tesler testified in Fulton County Superior Court that he was instructed by two other officers after the shooting to memorize a cover-up story that they had witnessed an informant buying drugs at the home of Kathryn Johnston.

But during often tearful testimony, Tesler said he eventually decided to cooperate with federal investigators because he "couldn't take it anymore."

The Rev. Al Sharpton, who was sitting in the courtroom, called the testimony "chilling."

Tesler is charged with lying in an official investigation, violating his oath as an officer and false imprisonment.

Johnston was killed Nov. 26, 2006, when she was shot 39 times as plainclothes narcotics officers busted into her house using a "no-knock" warrant. Johnston fired one shot from a pistol as police were breaking down her door, but she did not hit any of the officers.

Tesler's trial is likely the only one in the Johnston shooting because former officers Jason R. Smith and Gregg Junnier have already pleaded guilty to state manslaughter and federal civil rights charges.

The fatal shooting led to sharp criticism of the police department, a shake-up of the narcotics unit and a review of how officers obtain and use no-knock warrants, which are intended to keep drug suspects from having time to destroy evidence.