Mary Barra, CEO of General Motors, unveiled today the results of an internal investigation into the carmaker's handling of ignition switch problems, calling the report "brutally tough" and "deeply troubling."

"I hate sharing this with you," said Barra, who in January became the first woman to run one of Detroit's big three car companies. "Repeatedly, individuals failed to disclose critical pieces of information that could have fundamentally changed the lives of those impacted by a faulty ignition switch."

Related: GM is target of multiple probes over ignition recall. Watch: CEO apologizes to victims' families after recalls. Watch: 3 secrets you didn't know about Mary Barra.

In February, GM announced a recall of all 1.6 million cars from model years 2003-07.

According to depositions, GM engineers knew there was a problem with the ignition 10 years ago. And new evidence shows problems were flagged even earlier in 1999 with the engineer who designed the faulty switch.

The potentially fatal flaw in certain older GM cars has been linked to at least 13 deaths.

As a result of what GM termed "a pattern of incompetence and neglect," GM fired 15 employees - including senior executives, Barra announced today. Five others were disciplined.

The investigation found no signs of a cover-up and cleared Barra of any wrongdoing or knowledge of the issues, a point she'd maintained since the investigation started.

There is a specific group that goes through and looks at incidents, she told ABC News in a previous interview. "I was never a part of that process on this issue."

Sen. Richard Blumenthal blasted the report, though, saying "I don't trust GM to investigate itself. This report is the result of the best investigation that GM itself could buy."

Steve Berman, the attorney representing GM owners in a class-action suit, called the report "a cynical attempt at masking the truth."

"GM CEO Mary Barra seems to think that a public display of investigatory self-flagellation will somehow mollify or distract the public regarding what we believe is a much more insidious reality - that GM had full knowledge of the defects much earlier, and wantonly ignored the issue," Berman said in a statement.

Barra also announced today the creation of a victim's compensation program for those who lost loved ones or suffered serious physical injuries.

GM still faces five separate investigations, including one by the Justice Department, which experts said could lead to a fine similar to the $1.2 billion settlement paid by Toyota for its botched recall four years ago.

ABC News' Rebecca Jarvis contributed to this report.