Mary Barra addresses select reporters during a media briefing in Detroit.

General Motors is not fully cooperating with a federal probe of its defective ignition switches, investigators charged Tuesday.

In a letter to the beleaguered automaker, the lead attorney for the National Highway Traffic Safety Administration said GM was either "unwilling or unable" to answer more than a third of questions posed by regulators. Answers were due by April 3.

In response, NHTSA has levied a $7,000 per day penalty against the company. So far, the company has amassed a $28,000 fine. Perhaps more concerning for a company that posted a $3.2 billion profit in 2013 is that regulators say they'll ask the Department of Justice to compel responses for the missing information.

"To be clear, a complete response by GM means GM fully and substantively answers all questions and produces all responsive documents," O. Kevin Vincent, NHTSA's general counsel, wrote. General Motors disputes NHTSA's finding. A company spokesperson says GM has "fully cooperated" with the special order.

Intervention from the Justice Department would be a separate matter from the DOJ's own investigation into whether General Motors is criminally liable for its failure to recall more than 2.5 million affected cars in a timely manner. Documents have shown that GM knew about the problem as early as 2001 – four years before the cars even went into production. Yet the company didn't start to recall the cars until February. In the interim, GM acknowledges at least 13 people were killed in accidents related to the defect.

Two Congressional inquiries into the delay are underway, in addition to the pending DOJ and NHTSA investigations. In addition, an investigator general is conducting an audit of NHTSA's own inaction regarding the defect.

Testifying before the two Congressional subcommittees last week, General Motors CEO Mary Barra deflected many questions by saying she wanted to wait for the results of an internal GM investigation being conducted by former U.S. Attorney Anton Valukas before providing answers. Vincent's letter alleges GM attempted to answer many of the 107 questions in NHTSA's special order in the same manner: when investigators asked about the missing responses on April 4, GM cited the ongoing internal investigation as a reason for not fully responding, according to NHTSA.

"This was the first time GM had ever raised Mr. Valukas' work as a reason GM could not fully provide information to NHTSA in this timeliness investigation," Vincent wrote. "... Mr. Valukas' investigation is irrelevant to GM's legal obligation to timely respond to the Special Order and fully cooperate with NHTSA."

In month-long discussions with regulators, GM said it may not be able to fully provide answers to "technical engineering questions" by the deadline, and NHTSA stipulated it had no objection to the company taking extra time to ensure comprehensive responses to those particular questions. Yet when the responses arrived, they lacked information for not only technical engineering questions, but responses for "numerous" queries that were far more basic, Vincent said.

"These are basic questions concerning information that is surely readily available to GM at this time," he wrote. "Moreover, it is deeply troubling that two months after recalling the vehicles, GM is unwilling or unable to tell NHTSA whether the design of the switch changed at any other time."

General Motors said it has submitted 21,000 documents that total more than 270,000 pages in response to the NHTSA special order, and disputes Vincent's allegation that the company has not fully cooperated.

"GM has worked tirelessly from the start to be responsive to NHTSA's special order and has fully cooperated with the agency to help it have a full understanding of the facts," said Greg A. Martin, a company spokesperson. "We believe that NHTSA shares our desire to provide accurate and substantive responses. We will continue to provide responses and facts as soon as they become available and hope to go about this in a constructive manner. We will do so with a goal of being accurate as well as timely."

Pete Bigelow is an associate editor at AOL Autos. He can be reached via email at peter.bigelow@teamaol.com and followed on Twitter @PeterCBigelow.