NEW YORK, NY (REUTERS).- Some family members of victims of the September 11, 2001 attacks expressed outrage on Wednesday at a potential admission fee to a National Memorial Museum currently under construction at the former site of the World Trade Center towers.

Last week, president and chief executive of the National September 11 Memorial and Museum Joe Daniels told New York City council members about the possible $20 or $25 fee. He said the museum must generate enough income to operate, which memorial officials say could require between $50 and $60 million a year. The memorial portion will be free to enter.

"This is not the Metropolitan Museum of Art. It's supposed to be a memorial," said former New York City Fire Department Deputy Chief Jim Riches, whose son, also a firefighter, was killed in the attacks. "I think it's very crude."

"This was not supposed to be a money making thing. Most people will be coming to pay their respects to the people murdered that day," he added.

Sally Regenhard, who lost her 28-year old son Christian in the attacks, called the memorial and museum plans unnecessarily costly.

"We wanted a simple, beautiful memorial as a tribute to the greatest loss of life on American soil since the Civil War," she said. "What we have here is something that families never asked for."

"Our primary obligation is to make sure that we maintain the Memorial and Museum at the level that befits one of the most sacred sites in the country," Joe Daniels said in a statement on Wednesday. "We are exploring every avenue to raise funds to sustain this national tribute, including public funding and a suggested donation."

"Whether the Museum has a voluntary donation or admission fee will depend on the financial support we receive from other sources."

Additionally, the U.S. Mint is now selling a commemorative medal for $56.95, $10 of which goes to the museum.

Eleven family members of victims are on the board of the memorial and museum.

The price of admissions is just the latest spat between the memorial and groups of family members.

In early June, a group of 17 families of victims made a request under the Freedom of Information Law (FOIL) for a full list of next of kin of the 2,749 people killed in or around the Twin Towers to inform them and allow them to voice their views on a proposed plan to place more than 9,000 unidentified remains at the memorial.

(Reporting by Bernd Debusmann Jr.; Editing by Greg McCune)