The Wall Street Journal reported that the internal probe of General Motors Co.'s delay in recalling defective cars is expected to conclude there was no concerted coverup, but that managers operating in isolation failed to make connections and act on evidence of problems now linked to fatal accidents, people familiar with the situation said.

GM Chief Executive Mary Barra on Thursday is expected to outline steps to overhaul GM's culture and management, possibly including the departures of more employees, when she publicly presents the investigative findings by former U.S. prosecutor Anton Valukas, those people said.

The probe is expected to conclude that Ms. Barra, her direct reports and former CEO Dan Akerson were unaware before December 2013 of defective ignition switches installed in as many as 2.6 million cars, said people familiar with the findings.

Switches could abruptly slip out of the run position, disabling air bags and cutting the power assistance for steering and brakes. Documents provided to Congress by the nation's largest auto maker show that some of its employees knew about the problem as early as 2001, and that in 2004 and 2005 managers decided not to redesign the switches because of the cost.

Its expected findings of a weak corporate culture—and Ms. Barra's response to the report—won't be the last words on the recall. The U.S. attorney's office in Manhattan has a criminal probe under way, and states' attorneys general and the Securities and Exchange Commission separately are looking into GM's handling of the recall.

In May, GM signed an agreement with the U.S. Department of Transportation that it had violated federal laws requiring swift action to disclose defects and it had concealed information about those defects. It paid a $35 million fine, the maximum allowable by law, and agreed to U.S. monitoring of its recall responses for a year.

GM has linked 13 deaths directly to the defect, though lawyers for crash victims have criticized the way GM counted fatalities and the acting head of the National Highway Traffic Safety Administration has said the toll is likely higher.

Mr. Valukas' investigative findings are expected to take aim at what Ms. Barra has termed a "silo" culture in which managers in different departments failed to communicate safety concerns to one another or to senior executives. Documents released to Congress show various groups had pieces of information that could have led to the problem sooner, but they didn't share them with others.

Ms. Barra has taken steps to address the communications problems, reorganizing engineering operations and appointing a global safety chief who has access to the CEO's office and the legal department. More actions are expected to come out of Mr. Valukas' findings, people familiar with the situation have said.

In April, GM said it hired compensation expert Kenneth Feinberg for advice and a potential plan for dealing with accident victims in ignition-switch related crashes before and after its 2009 bankruptcy.

The company isn't obligated to address liabilities for incidents before its June 2009 bankruptcy filing under terms of its bankruptcy court agreement. GM has signaled it would look for a way to compensate pre-bankruptcy victims. Mr. Feinberg this week said his recommendations are still weeks away from being completed.

Thursday's release of the report marks one step in Ms. Barra's effort to quell the recall scandal and return the focus to its new vehicles. Since February, damaging revelations about the company's slow and incomplete responses to federal regulators, and a series of recalls to fix unaddressed defects in other vehicles, have overshadowed Ms. Barra's effort to win attention for new-model launches.

Lawyers for at least four victims of crashes involving recalled GM cars say they expect the report to provide details that would help them reopen wrongful death and injury cases that had been settled before the defects were disclosed. Their petitions are pending a bankruptcy court decision.

"GM failed to disclose underlying defect documents they had in their possession," said Bob Hilliard, a Texas lawyer representing two cases he is considering reopening. "The Valukas report, I hope, will provide a road map and bolster what is already known by answering who knew what and when."

The company said Ms. Barra "has expressed GM's regret and deep sympathy for all of those affected by the recall. We are determined to earn our customers' trust and to take actions necessary to make our safety processes world class."

Ms. Barra is likely to be called back to Capitol Hill as soon as this month to testify before a Senate panel to answer outstanding questions about the company's handling of the recall. In April, she deferred answering detailed questions about the recall before House and Senate panels, saying she would wait for Mr. Valukas' findings.

Congressional investigators and the families of crash victims also will be watching for what details the report offers on GM's lawyers knowledge of the connection between air bag failures and the ignition-switch defect.

"I'm going to be riveted to the report for how honestly it deals with the issue of lawyer responsibility here," said Sen. Richard Blumenthal (D., Conn.) "They not only enabled GM to lie to the federal government and to conceal wrongdoing and in effect defrauded the courts during the bankruptcy proceeding, but also they permitted the criminal justice system to, in effect, make the victims here victims twice."

GM declined to comment on his allegations.

Amid the recall gloom, GM's U.S. sales have cruised above the controversy. The auto maker's May sales jumped nearly 13% over a year earlier, much faster than the about 2% gain at No. 2 U.S. auto maker Ford Motor Co.

The better-than-expected May sales report helped send GM shares up $1.26, or 3.7%, to $36.52 in 4 p.m. New York Stock Exchange trading on Wednesday.