Graco Children’s Products is recalling almost 3.8 million car seats because faulty buckles can make it hard to free the child in an emergency, the company said on Tuesday.

But federal safety regulators said the recall, the largest in five years, did not go far enough, and have asked for an additional 1.8 million seats to be included because they use the same buckles.

In an unusual move, the National Highway Traffic Safety Administration told Graco in a letter that unless those additional seats were fixed it would take legal action to force a recall.

The seats in the recall are the 2009 to 2013 model years of the Cozy Cline, Comfort Sport, Classic Ride 50, My Ride 65, My Ride with Safety Surround, My Ride 70, Size 4 Me 70, Smartseat, Nautilus, Nautilus Elite and Argos 70.

The investigation into the seats began in 2012 after some parents complained to regulators that they had to cut the straps to remove their children.

One parent complained to the safety agency that it took 45 minutes to free her toddler and that worked only after the straps were loosened and the girl was squeezed between them. “It is extremely unnerving to have this happen to your child,” she wrote. “What if we had a car fire or a car accident?”

Graco told regulators that the problems with the buckles stemmed from…