Federal safety regulators expand their investigation of an air bag problem with Hyundai Elantras.

Back in May, the National Highway Traffic Safety Administration said it was taking a closer look at about 123,000 2012 Elantras.

This after receiving a complaint that a side air bag inflated in a crash and a metal bracket came with it, cutting a man's ear in half.

Now, the agency has added two more model years to the investigation...the 2011 and 2013 Elantras.

It's also upgraded the probe to an engineering analysis...which is one step closer to a recall.

Hyundai says the problem appears to be isolated.