JERUSALEM — A Human Rights Watch investigation published Thursday said an Israeli attack on a Gaza building in October had no apparent militant target, but killed 106 civilians, including 54 children, making it an “apparent war crime.”

International law prohibits attacks on military targets that will likely cause disproportionate harm to civilians. The Oct. 31 attack was one of the deadliest since the start of the war nearly six months ago.

Human Rights Watch says four separate strikes collapsed the Engineer’s Building in central Gaza, which was housing some 350 people, around a third of whom had fled their homes elsewhere in the territory.

Details of the Attack

Those killed included children playing soccer outside and residents charging phones in the first-floor grocery store, it said.

Thirty-four women, 18 men and 54 children were killed in the strike, according to the group, which says it corroborated its list of the dead with Airwars, a London-based conflict monitor. The dead came from 22 families. One extended family, the Abu Said family, lost 23 relatives in the strike, it said.

The Associated Press reported on four siblings who had been killed in the strike in October, including 18-month-old twin boys.

“They had no time here,” Sami Abu Sultan, their uncle, said a day after the building was destroyed. “It was God’s will.”

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Investigation and Response

While putting together the report, Human Rights Watch says it interviewed 16 people, including relatives of those killed in the attack, and analyzed satellite imagery, 35 photographs and 45 videos of the aftermath. It was unable to visit the site because Israel heavily restricts access to Gaza.

Witnesses told the rights group there was no warning ahead of the attack. Human Rights Watch says Israeli authorities have not published any information about the purported target and did not respond to its requests for information.

The Israeli military did not immediately respond to a request for comment from The Associated Press on Thursday.

Israel says it tries to avoid harming civilians and blames their deaths on Hamas because the militants operate in dense, residential areas. But the military rarely comments on individual strikes that kill dozens of people every day, including women and children.

Mounting International Criticism

Israel has faced mounting international criticism over its wartime conduct after its strikes killed seven aid workers earlier this week. Israel’s blistering campaign in Gaza has killed over 33,000 Palestinians, according to the Gaza Health Ministry, since the Hamas attack on Oct. 7 that killed 1,200 people and took around 250 hostage. The bombardment of the strip is one of the most intense aerial campaigns of the 21st century.