A U.S. judge on Thursday refused to let plaintiffs’ lawyers suing General Motors Co access notes from lawyers the company hired to prepare an internal report on the automaker's decade-long mishandling of a deadly ignition-switch flaw, reported Reuters.

U.S. District Judge Jesse Furman in Manhattan said interview notes from the “Valukas report” – named for Anton Valukas, chairman of law firm Jenner & Block, who GM hired to spearhead the investigation – were protected by attorney-client privilege.

While the ruling shields materials that could boost the cases by plaintiffs, they will be able to learn the identities of interviewed witnesses who were not named in the report.

Facing a backlash over its handling of the ignition-switch defect, GM tapped Valukas, a former federal prosecutor, last year to conduct a comprehensive review of why the company took so long to address the problem. The defect resulted in the recall of 2.6 million vehicles. A program to compensate victims has so far identified 45 deaths linked to the switch.

Valukas and his firm conducted more than 350 interviews with 230 witnesses, Furman wrote, and each lawyer took careful notes and prepared summaries of the conversations. The final report issued in June 2014 cited a series of missteps by GM employees, from lawyers to engineers, which allowed the problem to go unresolved for years.

The report is public, and GM has agreed to produce documents cited in the report to plaintiffs’ lawyers, who have sued on behalf of individuals injured or killed as a result of the switch, and customers whose vehicles lost value.

However, the company balked at turning over certain materials, including interview notes. GM said the notes were protected by attorney-client privilege because they were prepared by Jenner & Block lawyers. But plaintiffs’ lawyers argued that GM never intended to keep the Valukas report confidential, and that the notes were not protected because they were not legal advice.

Furman disagreed, saying GM had established a “valid claim” that the communications were privileged. And “the cost of withholding the materials is outweighed by the benefits to society of encouraging full and frank communication” between lawyer and client, he wrote.

Plaintiffs’ lawyer Steve Berman said he was disappointed with the ruling, but pleased to receive the list of unnamed witnesses. He also said plaintiffs believed the report was “flawed.”

GM did not immediately respond to a request for comment.