Gaya Kalderon last heard from half her family at 8:26 a.m. on Saturday morning.
“They are here,” her sister, Sahar, 16, wrote in a text message.
“Who is?” Gaya, 21, replied.
“We’re hiding from them,” Sahar said. “We left the house.”
“Where are you?” Gaya said. “Where are you going?”
There was no reply.
It wasn’t until Sunday that a terrified Ms. Kalderon saw any sign of her missing relatives, on social media. A video appeared of an Israeli child being shoved down a path by Palestinian militants.
“I am looking on Instagram and I see a video,” Ms. Kalderon recalled. “And it’s my brother.”
Erez, 12, and four other members of the Kalderon family are feared to be among an estimated 150 Israelis, many of them civilians, taken hostage by Palestinian militants during the broadest invasion of Israeli territory in 50 years. About 800 other Israelis were killed, according to a government statement.
The hostages were seized from homes in towns along Israel’s border with Gaza — including the Kalderons’ small village of Kibbutz Nahal Oz — as well as from military bases and an enormous outdoor dance party.
They include civilians, soldiers, people with disabilities, children, grandparents and even a 9-month-old baby. The hostages are also believed to include at least one Palestinian resident of Israel, a bus driver who spent the night near the outdoor party after driving Israelis there, his family said.
The capture of so many Israelis by Palestinian militants has taken the Israeli-Palestinian conflict into uncharted territory — not only in the sheer number of hostages, but also in the dire threats Hamas is making against them.
On Monday night, the Hamas military wing warned that it would execute a civilian hostage every time an Israeli airstrike hits Gazans “in their homes without warning.”
Since Hamas seized control of Gaza in 2007, gunmen there have kidnapped a handful of Israelis, including an Israeli soldier, Gilad Shalit, whose capture shook Israel deeply. He was freed in 2011, but only after prolonged negotiations and the release of around 1,000 Palestinians jailed by Israel.
The capture of a much larger number this weekend — and the fact that so many civilians were abducted — makes the standoff even more unpredictable and volatile.
Already, Israel has responded to the deadly assault by Hamas with a counterattack on the remaining gunmen inside Israel and an unusually intense series of strikes on Gaza, killing a total of about 687 Palestinians, according to the Gaza health authorities.
But the presence of so many Israelis in Gaza means that Israel risks killing its own citizens by doing so. On Monday, Hamas said that four Israeli hostages had been killed in an Israeli strike, though the claim could not be independently verified.
Then on Monday night, the Hamas military wing issued its threat to execute hostages if the strikes on Gaza continued.
Hamas may have hoped that taking dozens of captives would ease its chances of pursuing a broad prisoner exchange with Israel, said Eyal Hulata, who served as Israel’s national security adviser until January.
But for Israel, in the throes of one of the worst disasters in its history, now is not the time to even consider such an exchange, Mr. Hulata argued.
Mr. Hulata allowed that some Israeli captives might have been killed in an ongoing Israeli offensive. But that would likely be Hamas’s responsibility for “placing them as human shields,” he said.
“I want to bring everyone home. But we cannot do that as long as the other side thinks they can get away with this,” said Mr. Hulata.
In the past, Egypt and Qatar played key roles as mediators between Israel and Hamas as the two foes negotiated over captives. But for now, the sides were only “at the stage of passing messages along,” rather than direct talks to free prisoners, said Yaron Bloom, a veteran Israeli intelligence official who served for five years as the country’s point person for captured and missing Israelis.
“My assessment is that they are passing along this message: Hamas is responsible, and that if one hair on the heads of those elderly, women, babies, soldiers is touched — Israel will go ballistic,” Mr. Bloom said.
Yoni Asher’s nightmare began early on Saturday morning during a phone call with his wife, Doron Asher Katz.
Whispering, Ms. Asher Katz said that she, her mother and their two small daughters were trapped inside her mother’s safe room in a village near the Gaza border.
“She told me, ‘There are terrorists inside the house,’” Mr. Asher recalled.
Then came worse news: Ms. Asher Katz’s mother’s life partner, Gadi Moses, had left the safe room to try to reason with the intruders.
“She said, ‘they left — and they took him with them,’” Mr. Asher said.
Mr. Asher, 37, hoped that at least his spouse and children would be safe. But then the phone lines went dead.
It was the last time Mr. Asher heard from his wife.
Tracking her cellphone remotely, he saw that the device was taken on Saturday to southern Gaza, suggesting that she, too, had been kidnapped.
Then, he said, a video circulated on social media of abducted Israelis being driven through Gaza, bundled into the back of a pickup truck by armed gunmen. In the video, he said, a gunman attempts to spread a kind of blindfold over a woman’s head.
Mr. Asher said he recognized the woman. It was Doron, his wife.
His daughters Raz and Aviv, 5 and 3, and his mother-in-law, Efrat Katz, 67, were squashed alongside her, he said.
“I can’t sleep — I’m living outside my own body,” said Mr. Asher.
“I have two little babies, two little girls,” he added. “These little babies should not kept held or kept by terrorists.”
A few streets away on the same kibbutz on Saturday, Yarden Bibas, 36, and his wife Shiri, 30, also fled to the safe room in their home. Mr. Bibas was armed with a small pistol, he texted his family, while militants fired automatic weapons outside their windows.
“I love you all,” Mr. Bibas wrote. And then, 30 minutes later: “They’re coming in.”
The messages stopped.
Later, their family said they saw footage circulating on social media showing Shiri and their two brightly red-haired children — one of them just 9 months old — imprisoned by Palestinian militants.
There was no sign of Mr. Bibas.
His parents, Yossi Silberman, a artist who worked various jobs around the kibbutz, and his wife, Margit, a kindergarten teacher who suffers from Parkinson’s disease, also remain missing and are feared to have been abducted.
“I just hope that they are alive, and that they are together. And I want them home, with me, so I can hug them tightly again,” said Yifat Zeiler, the couple’s niece, sobbing.
“We feel that those responsible don’t know what to do, because this is a situation we’ve never been in before. That’s the feeling in Israel,” she added. “It’s a catastrophe.”
Patrick Kingsley reported from Jerusalem and Aaron Boxerman from London. Natan Odenheimer contributed reporting from Jerusalem.