The Israeli military said Wednesday it has closed its investigation into the shelling deaths of 21 members of a single Palestinian family and would not file any charges in what was one of the gravest incidents in the 2009 war in the Gaza Strip.

The military’s move, which exonerates Israeli soldiers from any responsibility in the killings, outraged relatives of the killed Palestinians and the Israeli human rights group that had pressed for the investigation. They said the findings proved the army is not capable of investigating the conduct of its soldiers.

“We are talking about a crime against civilians,’’ said Salah Samouni, 34, whose 2-year-old daughter was killed when Israeli shells slammed into the Gaza City house where the family had gathered.

“We know that God above will punish the killers. If they escaped trial, they can’t escape God’s punishment,’’ said Samouni, who survived the shelling.

Israel launched the three-week offensive in Gaza in response to months of rocket fire by the ruling Hamas militant group. About 1,400 Palestinians were killed in the fighting, including hundreds of civilians, and thousands of buildings were destroyed or damaged. Thirteen Israelis also died.

Surviving members of the Samouni family had claimed the family was ordered by Israeli soldiers to take refuge in a house that was then shelled, killing 21 people.

Israel has argued that Hamas was responsible for civilian deaths during the fighting, saying the militant group used schools, homes and mosques for cover while attacking Israeli troops.

In its findings, the Israeli military said its investigation “totally refuted’’ allegations that the incident was a war crime. It denied the building was deliberately targeted or that soldiers acted recklessly.