The probe covers My Ride and Nautilus car seats from 2009 through 2011 made by Graco Children's Products Inc. of Atlanta, the safety agency said in documents posted on its website Friday.
NHTSA has received 25 reports of problems from consumers, with some complaining that they had to use excessive force to unlatch the buckle. Three told the agency they had to cut the seat's harness to free their child, while six more said they had to pull the child through the harness. No injuries have been reported.
In one complaint from October of 2011, a parent wrote that they tried to get a 20-month-old boy out of a My Ride seat, but the center release button on the buckle couldn't be depressed. The parent was able to loosen the straps from the rear of the seat enough to free the child. "My biggest concern is that if this happens during an emergency, where we need to get him out quickly, we won't be able to without cutting the belt material," the parent wrote.
NHTSA does not identify people who file safety complaints.
The agency did not say how many car seats were affected by the investigation, saying in documents that the number was confidential. Investigators will decide if the seats should be recalled. They have asked Graco for information about other seats with the same harness parts.
A message was left for a Graco spokeswoman.
Graco's customer service number is (800) 345-4109.