Some of the passengers were in contact with the ground:
"According to flight attendants Amy Sweeney and Betty Ong, who contacted American Airlines during the hijacking, the hijackers had stabbed flight attendants Karen Martin and Barbara Arestegui and slashed the throat of passenger Daniel Lewin.Lewin, an American-Israeli Internet entrepreneur, had served as an officer in the elite Sayeret Matkal special operations unit of the Israel Defense Forces. Lewin was seated in 9B, and Suqami was directly behind him in 10B. The 9/11 Commission suggested that Suqami may have stabbed and killed Lewin after he attempted to stop the hijacking. Lewin was believed to be the first fatality in the 9/11 attacks."
"According to flight attendants Amy Sweeney and Betty Ong, who contacted American Airlines during the hijacking, the hijackers had stabbed flight attendants Karen Martin and Barbara Arestegui and slashed the throat of passenger Daniel Lewin.Lewin, an American-Israeli Internet entrepreneur, had served as an officer in the elite Sayeret Matkal special operations unit of the Israel Defense Forces. Lewin was seated in 9B, and Suqami was directly behind him in 10B. The 9/11 Commission suggested that Suqami may have stabbed and killed Lewin after he attempted to stop the hijacking. Lewin was believed to be the first fatality in the 9/11 attacks."