“GM did not respond to over a third of the requests in the special order by the April 3 deadline,” NHTSA Chief Counsel Kevin Vincent said in the letter, which is dated April 8 and was released to the public this afternoon. The fine now totals $28,000.

The letter adds to the complaints about GM’s answers to questions on the defective switch, which was used in several discontinued models, including the Chevrolet Cobalt and Saturn Ion. The switch, which could be bumped into the “accessory” position and deactivate the airbags, has been linked to crashes in which 13 people died.

GM, in a statement, said it 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.”

GM said today it has produced nearly 21,000 documents -- totaling over 271,000 pages -- through “a production process that spans a decade and over 5 million documents from 75 individual custodians” and additional sources.

“Even NHTSA recognizes the breadth of its inquiry and has agreed, in several instances with GM, to a rolling production schedule of documents past the April 3 deadline,” the company said Tuesday. “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.”

Several members of Congress voiced frustration at GM CEO Mary Barra during her testimony last week.

Barra declined to answer many questions about the ignition-switch flaw, saying the company had commissioned a full investigation by Anton Valukas, the chairman of the law firm Jenner & Block, and planned to wait for the results of that investigation before disclosing more information.

GM took a similar position in communications with NHTSA on April 3, the agency’s letter says.

“This was the first time GM had ever raised Mr. Valukas' work as a reason GM could not fully provide information to NHTSA," the agency says in the letter, before adding that GM again cited the Valukas investigation in communications on April 7. "Mr. Valukas' investigation is irrelevant to GM's legal obligation to timely respond to the special order and fully cooperate with NHTSA.”

According to NHTSA’s letter, GM said that it needed more time to answer some of the 107 questions because they would require detailed study by engineers. But NHTSA said the company also failed to provide answers to questions that should have required little analysis -- such as whether the ignition switch was redesigned any time besides 2006, when a GM engineer approved a change by supplier Delphi.

“These are basic questions concerning information that is surely readily available to GM at this time,” the letter said. “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.”