FANCY BOOSTERSInvestigators at the National Highway Traffic Safety Administration continue to take a heightened interest in car-seat buckles that are potentially defective.

Earlier this week, the Office of Defects Investigation upgraded its probe of buckles installed on Evenflo car seats, opening an engineering analysis of a problem in which parents and others say the buckles are difficult or impossible to open.

They're the same buckles at the center of an earlier Evenflo recall of 1,368,649 million car seats in April, and they're the same buckles at the heart of a rolling recall from fellow car-seat manufacturer Graco, which has reluctantly recalled 6.1 million seats related to the problem. Graco's recall is the largest in history related to car seats.

Evenflo specifically excluded its Embrace Rear Facing Infant car seats with the buckles from its earlier recall, according to NHTSA documents. The number of seats potentially affected is "confidential."

NHTSA says it has received 18 complaints related to the stubborn buckles. They echo the same complaint: It's nearly impossible to unlatch the buckle for unknown reasons.

In one of the complaints, filed in October 2012, a parent wrote that the five-point harness sticks, "and became nearly impossible to disengage. There is no food or liquid that is on the harness buckle that would cause this. ... We have decided to purchase a new car seat instead over safety concerns." In another, the complaint stated the person had to use pliers to unlatch the buckle.

An Evenflo spokesperson has not yet returned a request for comment.

Similar concerns were echoed in more than 6,100 complaints regarding the Graco car seats. Some parents and caregivers said they had to call emergency responders and/or cut the harness straps in order to remove children from the car seats.

Although Graco says it knows of no injuries or deaths attributed to the problem, one court case filed in California alleges a two-year-old girl died in a car fire because parents and other witnesses could not remove her from the car seat.