While Graco said it was not aware of any injuries resulting from the problem with the buckles, a letter from N.H.T.S.A. to Graco on Jan. 14 noted that it was a defendant in a wrongful death suit in Los Angeles County Superior Court over “the death of a 2-year-old child, Leiana Ramirez, who was killed in a car fire following a motor vehicle accident.” The girl, the agency said, “was seated in a Graco Nautilus car seat, of the subject seats.”

A Graco spokeswoman, Ashley Mowrey, wrote in an email, “The case was resolved pursuant to a confidential settlement agreement.” The recalled buckles were made by AmSafe Commercial Products of Elkhart, Ind. AmSafe did not respond to a request for comment.

The 1.8 million additional seats the safety agency wants recalled are rear-facing models used for infants. They are the Snugride, Snugride 30, Snugride 32, Infant Safe Seat-Step 1, Snugride 35, Tuetonia 35 and Snugride Click Connect 40.

Graco has insisted that those seats do not need to be recalled because if a buckle jammed, the seat containing the infant could be detached from the base, which is anchored to the vehicle, Ms. Mowrey said in a phone interview. She said parents who had trouble with buckles could get free replacements by calling Graco.

Image The company displayed old and new harness buckles on its website.

Clarence Ditlow, executive director of the Center for Auto Safety, a consumer advocacy group in Washington, said it was rare for a disagreement between a manufacturer and the safety agency to erupt in public and involve the threat of legal action.

“It shows a serious dispute,” he said.

The safety agency’s most recent letter to Graco took an unusually harsh tone. For example, the agency said that Graco’s response to the agency on Feb. 7 about its plan for the recall was “incomplete and misleading to both the agency and consumers.”