Visitors to the National September 11 Memorial & Museum must now pay a $2 service fee to reserve passes online or by phone.
The fee went into effect last month, although there is no charge for admission to the memorial on the World Trade Center site. There's also no charge for same-day passes distributed on a first-come, first-served basis.
Family members of some 9/11 victims say the fee violates the memorial's mission.
"They're making money off the people that died. It's disgusting," Jim Riches, a retired FDNY deputy chief who lost his firefighter son, told the New York Post.
Memorial President Joe Daniels issued a statement Sunday saying that, "like other similar institutions, in order to help support the operational needs of the 9/11 Memorial we have implemented a service fee, solely for advance reservations."
The memorial's website says the reservation system is temporary until certain construction projects are finished. Tax-funded grants have paid for about $300 million worth of construction, and more than $400 million came from private donations.
The memorial opened in 2011, attracting about 7 million visitors so far to its two reflecting pools with waterfalls that outline the footprints of the fallen towers.
The foundation that runs the memorial estimates that once the project is complete, the memorial and museum will together cost $60 million a year to operate.
The museum is still under construction after an interruption involving a funding fight between the foundation and the Port Authority of New York and New Jersey, which owns the 16-acre trade center site. Officials have said that the failure to open the museum on time has thrown off the foundation's financial planning.
Visitors to the exhibit space will see portraits of the nearly 3,000 9/11 victims, hear oral histories and view artifacts such as a staircase World Trade Center workers used to escape.