There's an old saying in Washington: When something goes wrong in government, it is far more likely to stem from incompetence than from conspiracy.

Now that familiar theme is being reprised by General Motors CEO Mary Barra, who cites "incompetence and neglect" by subordinates instead of a deliberate cover-up for the company's failure to address a fatal ignition flaw in Chevrolet Cobalts for more than a decade.

But that's a tough pitch to sell - and demands serious scrutiny by federal lawmakers.

Ms. Barra said she was relying on the findings of an independent investigation carried out at her request by Anton R. Valukas, a former U.S. attorney who's now a partner in Jenner and Block, a national law firm.

His report exonerated Ms. Barra and her top leadership colleagues of any responsibility for the failure of GM to issue a recall before this year.

However, while Mr. Valukas has an excellent reputation for thoroughness, the fact that Jenner and Block has been closely associated with GM over the years feeds suspicion that his report went too easy on the company's top executives.

Summarizing the report, Ms. Barra said, "The report highlights a company that operated in silos, with a number of individuals seemingly looking for reasons not to act, instead of finding ways to protect our customers."

Ms. Barra announced Thursday that she has fired 15 employees and disciplined five others for their roles in failing to address the Cobalt ignition issue.

She also announced a compensation plan for victims of the faulty Cobalts. GM acknowledges that 2.6 million cars were defective - and that there have been 13 deaths and 47 crashes caused by sudden shut-offs. But lawsuits against the carmaker allege as many as 60 deaths.

The Cobalt recall has been a nightmare for Ms. Barra in her first few months on the job. It also raises a question, not mentioned in Ms. Barra's review of the investigative report, of whether the issue ever came to the attention of her two CEO predecessors while GM was under federal ownership from 2009 through 2012.

Meanwhile, laments about "silos" can't change this: Some GM officials knew there were serious problems with Cobalt ignition systems long before the company acted to solve them.

That sounds much more like a cover-up than merely "incompetence and neglect."

It is reassuring to hear GM's boss pledge an overdue culture change to minimize future risks to customers.

But Congress still must thoroughly examine why it's taken so long for the company to chart this corrective course.