Federal safety regulators on Monday began an investigation into whether Graco Children's Products delayed reporting a safety defect that eventually led to the nation's largest recall of children's car seats.

After a series of disputes with the National Highway Traffic Safety Administration this year, Graco recalled about 6.1 million child seats.

The safety agency, under increasing scrutiny from Congress, has stepped up pressure on automakers and their suppliers over safety problems. The agency's deputy administrator, David Friedman, is set to testify on Wednesday at a congressional hearing looking into defective airbags made by the Takata Corporation.

At issue in the Graco seats are the buckles on restraining straps, which may be hard to unlatch, making it difficult to remove a child in an emergency. Graco resisted a recall, saying that there was no safety hazard and that any problems were caused because children spilled food or liquid on the buckles.

Federal regulators contended that was a foreseeable problem and a recall was needed.

The safety agency said on Monday that a review of documents provided by Graco indicated that the company was aware of complaints about the buckles as early as 2009 and that by 2012 the company was taking steps to address the problem. Those included telling owners how to clean the buckles and offering a lifetime warranty.

Under federal safety regulations, once a manufacturer is aware of a safety problem it must, within five business days, inform the safety agency or face a civil fine of up to $35 million, an amount some legislators are pushing to significantly increase.

"There is no excuse for delaying a recall to address any safety-related defect," Mr. Friedman said in a statement. "If Graco delayed in protecting children and infants from this defect, we will hold them accountable."

In a statement Monday, Graco said it "thoroughly analyzed all data related to the buckles and took the required actions to keep our consumers safe. We worked cooperatively with N.H.T.S.A. throughout its investigation and will continue to do so."

Graco, a division of Newell Rubbermaid, based in Atlanta, says on its website that "there have been no reported injuries related to this issue or the safe use of the car seats."

But among the documents gathered by the safety agency was a report of a wrongful-death suit in California 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 2011 motor vehicle accident." The girl, the agency said, "was seated in a Graco Nautilus car seat, of the subject seats."

Graco settled out of court, and a Graco spokeswoman, Ashley Mowrey, said earlier this year that the buckle was not at fault. She could not provide details because of a confidentiality clause in the settlement.

In 2005, Graco paid a $4 million fine to settle claims by the Consumer Product Safety Commission that it failed to inform the agency about problems with its products. Until late in October that was the highest amount paid to the product safety commission, when it was surpassed by a $4.3 million fine on Baja Inc., a minibike maker in South Carolina.