Nine years after the heroes of Flight 93 rebelled against the terrorists who hijacked their plane -- crashing it into a field in Shanksville, Pa., instead of allowing it to smash into an unknown, high-profile target in Washington, D.C. -- the design of a memorial to honor the 40 passengers and crew who died remains a subject of bitter controversy.

Since 2005, when plans for the Flight 93 National Memorial were unveiled, a group of critics, including the father of one of the heroes who died, have protested loudly that the memorial's design is rife with Islamic symbols. They haven't wavered in their protest -- even though some design elements have been changed -- and they plan to run a full-page ad opposing the design in a local newspaper on Friday and Saturday, when the nation will pause to remember September 11.

But the National Park Service says the critics -- whom it calls "conspiracy theorists" -- are expressing concerns that are baseless, and that it's already changed the design to remove any symbols that might be considered "Islamic."

At the center of the dispute is the Field of Honor, a circular, tree-lined landmass that will serve as the "heart" of the memorial, as well as a 93-foot Tower of Voices that will contain 40 wind chimes, one for each victim of the crash. Forty groves of red and sugar maple trees also will commemorate the victims, and ponds will be installed to serve as a natural barrier to the nearby Sacred Ground, the final resting place for the passengers and crew of Flight 93.

The newspaper ad -- paid for by Tom Burnett Sr., whose 38-year-old son died in the crash, and Alec Rawls, author of "Crescent of Betrayal: Dishonoring the Heroes" -- revives their claim that the memorial's Field of Honor clearly resembles an Islamic crescent and star, and that the entire site is orientated toward Mecca.

"A more obvious tribute to the terrorists is hard to imagine," reads the ad, which will be published in the Somerset Daily American and was provided in advance to FoxNews.com. "It is not surprising, then, that the giant crescent would turn out to point to Mecca, and be the centerpiece for the world's largest mosque."

Rawls says the planned memorial, which was created by Los Angeles-based architect Paul Murdoch, contains unacceptable, intentional Islamic symbols, including the giant crescent he sees in the Field of Honor, a 150-acre bowl-shaped landform that will have missing trees in two places to mark the path of the airliner and the crash site.

"All they did was disguise it, nothing's changed," Rawls said, referring to the alterations that have been made to the design, including changing the title of Crescent of Embrace to Circle of Embrace, since the objections were initially raised.

Rawls acknowledged that trees were added to the design so as to not create the much-debated crescent, but he insisted that the site is still orientated toward Mecca, and he wants a congressional hearing into the matter.

"Intentional or not, these features are entirely unacceptable," reads a two-year-old online petition that has more than 10,000 signatures. "This travesty must stop and investigations must begin … We ask that the crescent design be scrapped entirely and that it be replaced with a new design that is not tainted by Islamic or terrorist memorializing symbolism. We demand a fitting and proper memorial that HONORS the brave men and women of Flight 93."

If left unchanged, Rawls said, the memorial will ultimately serve as a government-built "Ground Zero mosque" in Pennsylvania. "This is the architect's plan," he told FoxNews.com. "This is what he wanted. To not recognize what the architect is doing here is nuts. This is state establishment of religion."

But the group that is building the memorial says that the design is nothing of the sort, and that it has grown weary of the argument.

"The Flight 93 National Memorial partners have said all there is to say about the conspiracy theorists," Joanne Hanley, the National Park Service's superintendent of the Flight 93 Memorial, told FoxNews.com. "We will not re-engage with these people."

According to one of several official websites dedicated to the project, the continuous sound of the chimes will serve as an "audible reminder of the selfless acts of courage" displayed by passengers and crew.

But the newspaper ad notes that the tower is "cut at an angle at the top so that its crescent arms reach up into the sky. Crescent-topped minarets are a daily sight across most of the Islamic world. Is this really a proper marker for the victims of Islamic terrorism?"

Burnett, whose son Tom died along with 39 others in the crash of Flight 93, was unavailable for comment for this article, but the retired high school English teacher told The New York Times in 2008 that he voted against the design.

"It's really revolting to me, this whole thing," he told the newspaper. "It's an insult to my son and all the others."

Messages seeking a response from Murdoch were not returned, but National Park Service (NPS) officials say the criticisms have no validity.

Hanley referred further inquiries to a section of the memorial's website that pertains to questions regarding the design, which was selected from more than 1,000 entries from 48 states and 27 countries.

"This memorial solely honors the heroic actions of the 40 passengers and crew, who defeated the terrorists," the site reads. "The intent of the architect was to honor the passengers and crew. When questions were raised about the design, they were taken very seriously."

NPS officials and Flight 93 partner organizations later concluded that the memorial design does not imply or depict religious iconography, according to the site, which also referred to a 2008 statement from the president of the Families of Flight 93 in full support of the design.

"These allegations are hurtful to the overwhelming number of families fully in support of the memorial," read the statement by Gordon Felt, whose brother, Edward, died in the attacks. "Furthermore, they are misleading and not based in fact."

Felt's statement continued, "Over the past few years, we have reached out to experts in the areas of religious iconography, Islamic Law/theology and memorial design. The findings of these experts are clear and unwavering in that people looking for controversy will find it if they look hard enough regardless of the circumstances."

Another website connected to the memorial, honorflight93.org, allows visitors to donate money to the $58 million project. The National Park Foundation, the official charity of America's national parks, released a public service campaign late last month in hopes of raising $18 million to complete the project.