The National Highway Traffic Safety Administration said it's investigating whether Hyundai and Kia met all federal requirements in conducting safety recalls involving more than 1.6 million vehicles with engine problems.

The probe will focus on the timeliness and scope of the recalls and compliance with reporting requirements, NHTSA said. The recalls involve Theta II engines at risk for seizing.

Back in September 2015, Hyundai Motor America filed a defect report with NHTSA and recalled about 470,000 2011- and 2012-model year Sonata cars with the Theta II engine. In the defect report, Hyundai blamed the trouble on manufacturing debris and a machine process that created an uneven surface roughness that impeded oil flow to connecting rod bearings. The problem can eventually lead to premature bearing wear and engine seizing.

At the time, Hyundai limited the recall to vehicles produced on or before April 12, 2012. The automaker indicated a manufacturing process change in April 2012 resolved the debris issue.

But on March 31 of this year, Hyundai expanded the recall to include about 572,000 2013- and 2014-MY Sonata and Santa Fe Sport vehicles with Theta II engines. At the same time, Kia Motors America filed a defect report to recall 618,160 2011-2014 Optima, 2012-2014 Sorento, and 2011-2013 Sportage vehicles for the same Theta II engine issue.

In the defect report, Kia stated that it didn’t conduct a recall back in September 2015 because Kia’s Theta II engines were manufactured on a different production line than the recalled Hyundai vehicles.

In NHTSA’s investigation notice, the agency pointed out that Hyundai has continued to attribute the problem to manufacturing debris.

Kia is partially owned by Hyundai.