Thierry Saada (see here for a tribute) worked for Cantor Fitzgerald on the 104th floor of the World Trade Center. On the morning of September 11, 2001, he called his wife from his office to wake her at 8:20. After the planes hit, she called his cellphone and received no answer, but several minutes later he called her back to tell her that the building was being evacuated. She received no further contact from him after that time. Email records show that he sent an email from his office at 8:21 that morning. Given the fact that his body was never found, can we presume that he is dead and his wife is permitted to remarry?
Rav Ovadiah Yosef's response to this painful question presumes almost from the get-go that his wife will be permitted to remarry. The Gemara in Yevamot 121b says that if a person falls into a fiery furnace we can testify that he is deceased - and certainly that standard should apply to this case, where someone was located above the raging fire, which experts have testified likely resulted in many deaths from smoke inhalation or burning before the buildings collapsed. Rav Yosef notes that we do not follow the view of the Yerushalmi brought by Tosafot that states that we can hope that a miracle happened similar to what happened to Chananiah, Mishael, and Azariah when they were saved from the furnace. Rather, if all factor point to the fact that the person has died, we can assume that to be the case and permit his wife to remarry.
The next section of the teshuva deals with issues of rov, and to what extent we can be strict or lenient in the presence of multiple majorities (such as the fact that a majority of people on those floors did not survive, and that a majority of survivors made contact with their relatives), and whether or not it matters if they occur simultaneously.
Rav Yosef then notes an interesting and logical view of the Chatam Sofer, who states that while during the time of the Gemara we were perhaps concerned that someone who was missing might resurface even after a long time, nowadays with the advent of sophisticated mail systems we would assume that we would hear from someone who had gone missing. Even more so in our days, says Rav Yosef, when we have telephones and newspapers and consulates and all sorts of ways for someone to make contact with his family. Furthermore, as Mr. Saada's wife was in her 9th month of pregnancy, and the child born two weeks later had both a brit mila and a pidyon ha-ben, it would stand to reason that if he were alive, Mr. Saada would have tried to get in touch with his family in some way. The fact that he did not serves as ample proof to his tragic demise. While there are those who reject the thesis of the Chatam Sofer (and indeed there are still cases today where people resurface after a long absence), Rav Yosef feels that it may certainly be accepted in this case, certainly when combined with the extreme likelihood that no one survived if they were on the floors above where the planes hit (and especially since emergency rescue squads combed the wreckage of the buildings for weeks and found no one).
Finally, Rav Yosef notes at the end of the teshuva that one many rely on the sound of a voice over the telephone as reliable proof that a person existed. He cites sources that permit a husband to order a get to be written via telephone and that accept testimony that a man died based on the witnesses recognizing his voice. As such, the fact that the husband spoke with his wife after the planes hit is sufficient for us to establish that he was, in fact, in the building (in other words, we trust that the wife knew that she was speaking with her husband and that he was truthful in claiming that he was in the building. I would note that since he called on a cellphone, it is harder to use cellphone records to pinpoint his location. Had he called from an office phone, which likely was not working at that point, the call could have been traced, thus providing a more reliable electronic record.).