The U.S. Department of Health and Human Services says Gov. Chris Christie's administration failed to submit a new plan Thursday for how it would use the money. Under the rules of the program, that inaction means the grant could be taken back.
But authorities have not said definitively whether that is the case.
The money was awarded two years ago to help New Jersey set up an insurance marketplace. But Christie opted to join the federally run exchange instead.
New Jersey officials asked this week for more leeway on how to spend the money after the federal government rejected its plans.