The last memories many of the relatives have of the passengers and crew of hijacked United Flight 93, which crashed in a Pennsylvania field on Sept. 11, 2001, are the sound of their loved ones' voices on cell phones.

Because of that, sound is an integral part of the memorial planned in the bowl-shaped field in Somerset County near Shanksville. A "Tower of Voices" bearing 40 wind chimes -- representing the 40 passengers and crew members who gave their lives trying to wrestle the plane away from the hijackers -- will be erected at the site.

The chimes will be of various sizes and tones. "They're trying to respect the collective and individual efforts that came together that day based on the phone calls and cockpit recording," explained Hamilton Peterson, president of Families of Flight 93, whose father and stepmother were passengers on the doomed flight.

Trees will be planted in groves to commemorate all who died that day, and wildflowers will mark the area known as the sacred ground where the passengers and crew are laid to rest.

A break in the tree line will mark the path the airplane took.

Flight 93 was headed from Newark to San Francisco when four hijackers took control of the aircraft and turned it toward Washington, D.C., where many believe the target was the Capitol or the White House.

The airliner was the only one out of four hijacked that day that did not make its mark.

Because of heavy runway traffic in Newark, Flight 93 took off 41 minutes late, giving the associates and relatives of those aboard time to tell them about the two other aircraft that had crashed into the World Trade Center in New York City. Later, a third airplane rammed into the Pentagon in Arlington, Va.

In those locations, large memorials have been planned and replanned in the ensuing five years. The rebuilding of the World Trade Center will incorporate a memorial, and the Pentagon's remembrance will feature 184 cantilevered benches and reflecting pools for those who died at that location.

The planners of the memorial in Pennsylvania, meanwhile, took into account the distinctive landforms and sweeping landscape of the field. A single building or monument would have been lost in the broad expanse, so the designers opted for a memorial park of sorts with the final resting place of the passengers and crew as the focal point, said designer Paul Murdoch of Paul Murdoch Architects in Los Angeles.

"We took the land form known as the bowl ... the sacred ground is to the side and near the bottom of that bowl. And we wanted to create this gesture of embrace that formed an edge to that bowl and focused the public to that sacred ground," Murdoch said.

A sloped wall will allow the public to look into the crash site and leave tributes. Currently, a 40-foot chain-link fence serves that purpose. More than 500,000 people from around the world have visited the site since Sept. 11, 2001, according to a July 2006 report from the government-appointed Flight 93 memorial commission.

Allowing visitors to walk around the 2,200-acre site and participate in all its elements was important to the design, said Murdoch. When people arrive at the wind chime tower, they'll be able to drive down a winding road to the edge of the bowl, get out of their vehicles and follow a walkway marking the flight's path.

"They'll be walking through gaps in walls that frame the sky where the plane went through the site overhead. And so it brings the visitor right in focus of this event," he said.

Part of the memorial is an oral history project aimed at documenting the day for research and educational purposes. The National Park Service has collected more than 100 audio interviews from family and friends of the passengers and crew, first responders, official agencies involved in the investigation and recovery and members of the community, according to the commission's report.

Peterson said, on a personal level, he hopes at the very least a transcript of the cockpit recording will be available as part of the memorial's interpretive material, and possibly the 911 recordings if the family members agree to their release.

The recordings "would bring to life the incredible courage of that day," he said.

The Flight 93 families were consulted throughout the planning process and helped choose the design they thought would best honor the passengers and crew, said Peterson.

"We'd like this memorial to stand in eternity to provide a beacon of courage and an example of ordinary citizens rising to the challenge" of those seeking to do harm, including terrorists, he said.

Peterson cited the airline passengers who fought against Richard Reid, known as the "shoe bomber," and the Muslim neighbor who reportedly tipped off police in Britain about the plot to detonate liquid bombs on U.S.-bound airliners as a direct result and a tribute to the passengers who took on the hijackers on Flight 93.

"We can no longer rely on the military, it's up to all of us," he said.

After a formal planning process ends in the fall of 2006 and the land is acquired from private owners, construction of the $58 million memorial will begin. The target completion date is 2011, the 10-year anniversary of Sept. 11.

-- By Larisa Epatko, Online NewsHour