The NHTSA will attempt to determine if the campaign fell short in its timeliness or scope.

The National Highway Traffic Safety Administration has launched an investigation into Hyundai-Kia's recall for 'Theta II' engines.

The company recalled more than a million vehicles to address a manufacturing screwup that left debris in some engines, potentially restricting oil flow and causing engine failure.

"Hyundai limited the population of recalled vehicles to those produced on or prior to April 12, 2012, stating that a process change in April 2012 resolved the issue of manufacturing debris," the NHTSA says. "On March 31, 2017, Hyundai expanded its original recall to include 572,000 MY 2013-2014 Sonata and Santa Fe Sport vehicles with 'Theta II' engines ... The DIR continues to describe the defect as an issue involving manufacturing debris."

The NHTSA is apparently investigating the gap between the 2015 recall and the 2017 campaign related to the same manufacturing defect.

The Theta II engines were used in Hyundai's 2011-2012 Sonata and Kia's 2011-2014 Optima, 2012-2014 Sorento and 2011-2013 Sportage.