Though long used to war, Israel was blindsided by the multi-pronged attack by Hamas gunmen in which hundreds were killed.

Not only was the number of those unaccounted for unknown, but many were believed to have been taken captive into Gaza -including young children - with others hiding in their besieged Israeli villages.

The Gaza border region, which is just an hour's drive from the police information center in Airport City, remained sealed off by the military as troops continued street-by-street battles with gunmen, learning with each stage the extent of the carnage.

One mother spoke with Reuters after giving her DNA sample and handing over a brush with hair follicles from her son, who was at an outdoor dance party that was stormed by Hamas gunmen. She declined to give her name.

"The last we heard from him, he was calling from the car, saying he was trying to get away and that they were shooting at him," she said. "I'm doing whatever I can to find out what happened."

Outside the police center, a man prayed with a rabbi as his fiancee filled in a report for her missing brother.

Yoni Asher, who was back at his home in central Israel after countless attempts to get assistance from authorities, said his wife was visiting her mother in the community Nir Oz near the border with their two young daughters when Hamas attacked.

"She told me the terrorists are in the house," he said. Then the call got cut off.

Later on he located her mobile phone through her Google account. Its location was Khan Younis, a city in Gaza. He then saw a video that spread on social media of them being taken to Gaza.

"I surely identified my wife, my two daughters and my mother-in-law on some kind of a cart, and terrorists of Hamas all around them," he said.

"My little two girls, they are only babies, they are not even 5-years-old and 3-years-old ... I don't know in what terms they are captive. I don't know what happened to them."

(This story has been refiled to fix the spelling of 'fiancee' in paragraph 7)

(Additional reporting Miro Maman; Editing by Hugh Lawson)

By Dan Williams