General Motors boss Mary Barra will testify April 1 before a congressional committee investigating why the largest US automaker failed to react for years to a defect linked to 12 deaths. The chief executive is expected to face a grilling from lawmakers over why it took the US auto giant until last month to recall 1.62 million vehicles in North America, more than a decade after the problem was first detected. Barra, who took the reins of the company in January, and acting head of the National Highway Traffic Safety Administration David Friedman will testify before an oversight panel of the House Energy and Commerce Committee. "Their testimony is critical to understanding what the company and NHTSA knew about the safety problems, when they knew it, and what was done about it," committee chairman Fred Upton and congressman Tim Murphy said in a statement Thursday. GM is the focus of multiple investigations by US authorities for being too slow to react to a defective ignition switch it linked to 31 accidents and 12 deaths in its 2005-2007 Chevrolet Cobalt and 2003-2007 Saturn Ion models. The problem was detected as early as 2001, but GM waited until last month to issue its recall. NHTSA is also under the gun for not acting despite being aware of complaints that the ignition would turn off while the cars were in motion. GM suffered a further setback Monday when it recalled an additional 1.8 million vehicles linked to three new problems. Barra responded forcefully, announcing she would release an "unvarnished" report following a thorough review and vowing to improve how GM handles defect reports. In a recent video statement to employees, Barra called the federal probes "serious developments that shouldn't surprise anyone." "After all, something went wrong with our process in this instance and terrible things happened," she said. Barra, 52, is the first woman to lead a major automaker.