The public should take no comfort in the announcement that Boston schools Superintendent Carol Johnson is investigating the work record of former high school headmaster Rodney Peterson. Since the day she hired Peterson in 2009, Johnson has shown consistently bad judgment in all matters regarding the disgraced headmaster.

A Globe review of payroll records shows that Peterson missed more than 20 percent of the last school year at the O’Bryant School of Mathematics and Science. Yet Johnson seemed none the wiser about his absenteeism. Even when confronted directly with Peterson’s misdeeds, Johnson ignored them. She failed, for example, to discipline Peterson after he was arrested last year for assaulting his wife. Inexplicably, she provided him with a character reference at his sentencing hearing.

An independent investigation — not an inside job — is needed to address the issues raised in the Peterson case. And a significant part of that investigation should focus on why Johnson championed such a poor performer. Neither Johnson nor anyone who works for her can be trusted with that task.

Peterson worked at both the Odyssey and O’Bryant high schools. He came highly recommended by Shonda Huery, Johnson’s assistant chief academic officer at the time. Peterson and Huery were dating, a fact that Johnson says she didn’t know until long after she hired him. Johnson also says she knew Peterson only by reputation when both were working at the Memphis school department. Huery also worked in Memphis, where she started her career as an intern to Johnson.

The beginning of the school year is just a few weeks away. The last thing the city needs is a cloud of suspicion hanging over the school department. It shouldn’t require that much time to determine how someone with Peterson’s work habits and temperament not only wormed his way into the system but received promotions to boot. And nothing short of an independent investigation can be trusted to provide those answers.