Graco is recalling nearly 3.8 million car safety seats because children can get trapped by buckles that may not unlatch. But the company has drawn the ire of federal safety regulators who say the recall should include another 1.8 million rear-facing car seats designed for infants.

The recall covers 11 models made from 2009 through 2013 by Graco Children's Products Inc. of Atlanta. It's the fourth-largest child seat recall in U.S. history, according to the National Highway Traffic Safety Administration, the government's road safety watchdog.

The agency warned that the problem could make it "difficult to remove the child from the restraint, increasing the risk of injury in the event of a vehicle crash, fire or other emergency."

NHTSA also criticized Graco in a sternly-worded letter dated Tuesday, saying the recall excludes seven infant car seat models with the same buckles. Both the company and NHTSA have received complaints about stuck buckles on the infant seats, the agency said.

"Some of these consumers have had no choice but to resort to the extreme measure of cutting the harness straps to remove their child from the car seat," the NHTSA letter said.

The agency wants Graco to identify the total number of seats that potentially have the defect and explain why it excluded the infant seats. NHTSA, which began investigating the seats in October of 2012, said the investigation remains open. The agency said it could hold a public hearing and require Graco to add the infant seats.

Graco, a division of Atlanta-based Newell Rubbermaid, told The Associated Press that its tests found that food or beverages can make the harness buckles in the children's seats sticky and harder to use over time.

Rear-facing infant seats aren't being recalled because infants don't get food or drinks on their seats, Graco spokeswoman Ashley Mowrey said. But Mowrey said Graco will send replacement buckles to owners of infant seats upon request.

Mowrey said the company has issued cleaning tips for the buckles, and began sending replacement buckles to owners last summer. Graco is also sending instructions for how to replace the buckles and posting a video on its website to show parents how to replace them.

In documents sent to NHTSA, Graco estimated that less than 1 percent of the seats involved in the recall have had buckles that were stuck or difficult to unlatch.

Mowrey said there have been no reported injuries due to the defect.

Parents should check seat buckles and contact Graco for a free replacement, NHTSA said. The agency also said people should get another safety seat for their children until their Graco seat is fixed.

NHTSA, in the letter to Graco, also accused the company of soft-pedaling the recall with "incomplete and misleading" documents that will be seen by consumers. The agency threatened civil penalties and said that Graco should delete from its documents "any statements that may lead the public to discount the seriousness of the safety risk presented by this defect."

In addition, NHTSA said that last month, it started investigating four models of Evenflo child safety seats, which have a design similar to the recalled Graco seats and may use buckles made by the same manufacturer, AmSafe Commercial Products Inc. of Elkhart, Indiana.

"NHTSA is also in contact with AmSafe to identify any additional child seat manufacturers that use harness buckles of the same or similar design," NHTSA's statement said.