JERUSALEM—The Israeli military says it has closed its investigation into one of the gravest incidents in its 2009 war in the Gaza Strip and has not found any soldiers responsible for the deaths of civilians there.

In January 2009, 21 members of a single Palestinian family were killed after Israeli shells slammed into the house where they had gathered.

The Israeli military rejected allegations that the incident was a war crime. It denied the house was deliberately targeted or that soldiers acted recklessly.

B'tselem, an Israeli human rights group, said it was "intolerable" that the military exonerated itself of responsibility in the case.