For 16 months, the smiling faces of Shiri Bibas and her two young sons, Ariel and Kfir, had been slowly receding into the background of Israeli life as their photographs — posted on walls and bus stops soon after the family’s abduction to Gaza in October 2023 — began to fade, tear and peel.

On Friday, the Bibas’s lives and disturbing deaths were swept back to the forefront of Israel’s collective consciousness in such a startling and unsettling way that it set off fresh alarm about the fate of the fragile cease-fire in Gaza and the high-level diplomatic efforts, which had gathered momentum in recent days, to extend the truce and end the war.

Early on Friday morning, the Israeli military announced that the body of Ms. Bibas — nominally returned, along with those of her sons, by Hamas to Israel on Thursday — appeared to be that of someone else. And an autopsy of the two boys, aged 4 and 8 months at the time of their abduction, revealed that they were killed by terrorists in Gaza, the military said.

Hamas, which had previously said they were killed in an Israeli missile strike, said in a statement that it was investigating the claims and suggested that Ms. Bibas’s body might have been mistakenly confused for that of a dead Palestinian in the chaotic aftermath of an Israeli attack. Neither side’s account could be independently verified.

The news set off a paroxysm of fury and agony in Israel rarely seen since the tumultuous days that followed Hamas’s attack on Israel on Oct. 7, 2023, when up to 1,200 people were killed and 251 were abducted, including Ms. Bibas and her sons, on the deadliest day in Israeli history.