Documents obtained by investigators for a US Congress-mandated probe claimed that General Motors Co decided to not implement a proposed fix on ignition switches on Chevrolet Cobalt models in 2005 after months of studying. It has been known that a project engineering manager deemed the plan too expensive when implemented on a massive scale, Bloomberg said.

The news agency also said that the automaker had another opportunity to implement the proposal, but failed to do so even after the National Highway Traffic Safety Administration recommended the fix in 2007. This was according to an interview conducted between staff members of the the House Energy and Commerce Committee and NHTSA officials.

Tomorrow and on April 2, congressional hearings will be focusing on GM's decisions to not pursue a fix and the company's eventual recall of 2.6 million affected cars for the same ignition issue. The automaker's chief executive officer, Marry Barra, and acting Administrator of NHTSA David Friedman, will be asked in the hearings on how GM had managed several years of complaints from customers about stalling cars and disabled air bags , which are now being linked to the faulty switches and 13 deaths.

Bloomberg said NHTSA opted to not launch an official investigation about the defect in 2007 after reviewing the data on the disabled air bags. The news agency posit that NHTSA's inaction in 2010 helped convince the investigative committee in the US Congress about the need for the agency to provide its testimony about the car defects.

NHTSA spokesman Nathan Naylor issued a statement about the agency's questionable decision in 2007, "As we have stated previously, the agency reviewed data from a number of sources in 2007, but the data we had available at the time did not warrant a formal investigation. Recent data presented by GM provides new information and evidence directly linking the ignition switch to the air-bag non-deployment. That's why we are aggressively investigating the timing of GM's recall."

Barra has since apologized for the company's slow response to the defects, Bloomberg said. GM spokesman Greg Martin added in an emailed statement, "We deeply regret the events that led to the recall. We are fully cooperating with NHTSA and the Congress, and we welcome the opportunity to help both have a full understanding of the facts."