A dual US-Israeli citizen believed to have been abducted by Hamas was killed during the 7 October raid in Israel, the White House said on Friday.
Gadi Haggai, 73, was reportedly taken alongside his wife Judi near Kibbutz Nir Oz in southern Israel on 7 October.
It is believed his remains are being held by Hamas in Gaza. It is unclear exactly how or where he died.
Five to 10 US citizens are still being held hostage, according to the Hostage and Missing Persons Families Forum.
President Joe Biden said in a statement that he was "heartbroken by the news that American Gad Haggai is now believed to have been killed by Hamas on October 7".
He said he grieves for the Haggai family, and called their suffering "intolerable".
"I reaffirm the pledge we have made to all the families of those still held hostage: we will not stop working to bring them home," the president added.
Mr Haggai, a retired chef and musician, was walking with his wife near Nir Oz on 7 October when the initial Hamas attack on southern Israel took place.
He was described as a "man full of humour who knew how to make those around him laugh", as well as a gifted flautist and "musician at heart" by Liat Bell Sommer, a spokesperson for the Hostage and Missing Persons Families Forum - a group established by the families of those abducted by Hamas.
During the attack, his wife alerted friends and family members that they had both been wounded. There has been no contact with the couple since.
"She said they were shot by terrorists on a motorcycle and that my dad was wounded really bad," the couple's daughter Iris Weinstein Haggai was quoted as saying by the Times of Israel. "Paramedics tried to send her an ambulance. The ambulance got hit by a rocket."
Approximately 240 hostages were taken during the Hamas raid on Israel on 7 October, which left 1,200 people dead.
According to Israeli government statistics cited by Reuters, 129 people remain in Gaza, including 22 who are dead. The rest of the hostages were repatriated during a brief November truce or have been recovered during Israeli military operations.
The forum believes that between five and 10 of the remaining hostages hold US citizenship. Earlier this week, the White House said it believes eight Americans are still in Hamas captivity.
In a White House briefing earlier this week, US National Security Council spokesman John Kirby said that the US is in "serious talks and negotiations" over another humanitarian pause and hostage exchange.
Hamas, for its part, has ruled out any more hostage releases until Israel agrees to a "full cessation of aggression".