Family members who lost relatives on the four planes that were hijacked on Sept. 11, 2001 (search), gathered for a closed-door briefing on Friday to hear audio tapes of phone calls between passengers and family members or co-workers on the ground.

Justice Department (search) letters sent to family members said they would be able to listen to tapes of cell phone calls from the flights, including calls made by American Airlines Flight 11 flight attendants Betty Ong and Amy Sweeney before their plane hit the World Trade Center (search).

It was not immediately clear what other taped phone calls investigators possess, or if the family members would hear all of the tapes the government has. The briefing was scheduled to begin at 12:30 p.m. and last about two hours.

Among those arriving at the hotel Friday morning was Sandy Dahl of Denver. Her late husband, Capt. Jason Dahl, was the co-pilot of United Flight 93, which was bound from Newark, N.J., to San Francisco when it was hijacked.

The plane crashed in a western Pennsylvania field after passengers were believed to have fought with the hijackers.

"It is tough, but I'm hungry for the information," said Dahl, who chairs the board of Families of Flight 93. "I need to know. I need to know everything that can be known."

Family members had asked for access to the phone calls and other evidence after some of it was revealed during recent hearings of the independent commission investigating the attacks.

"We are here to make sure today that we learn the truth," Dahl said. "We're also here joining together to remind the nation that this was a victory, this was a victory on 9/11. Our loved ones were the only thing that didn't go completely wrong."

Security was tight outside the meeting site, a Radisson Hotel about five miles north of Princeton. NBC News reported that family members attending the briefing session would be required to sign a nondisclosure agreement.

Prosecutors in the case of Zacarias Moussaoui (search), the only person charged in the United States as part of the Sept. 11 conspiracy, were to conduct the briefing. The material being presented is part of the evidence gathered during the federal investigation into Moussaoui's role in the hijackings, the letter to relatives said.

Ong's tape was played in public in January at a hearing of the Commission on Terrorist Attacks Upon the United States.

"The cockpit is not answering their phone," Ong told the American Airlines operations center. "There's somebody stabbed in business class, and we can't breathe in business. Um, I think there is some Mace or something. We can't breathe."

Nearly 3,000 people were killed when the hijacked aircraft crashed into the World Trade Center, into the Pentagon and in Pennsylvania. Not counting the 19 hijackers, 246 passengers and crew members aboard the planes were killed.

A second invitation-only briefing for family members has been scheduled for July 14 in Boston. That session also will be broadcast via closed-circuit hookup to sites for family members in Washington, D.C., and Los Angeles.