(AP) Federal safety investigators say General Motors failed to respond to more than one-third of its requests for information about a faulty ignition switch linked to 13 deaths by an April 3 deadline.

In a letter to GM’s top lawyer sent Tuesday, the National Highway Traffic Safety Administration imposed its maximum allowable fine for the delay of $7,000 per day. That adds up to $28,000 so far, and the fines will continue to accrue until GM responds.

According to the letter, GM was granted extra time to answer “technical engineering questions” posed by the agency. But NHTSA contends many of the questions GM failed to answer are not engineering ones, and should have been answered by the deadline.