'Unrelated' to Goldstone
The army said it opened the investigation after the incident was brought to its attention by the United Nations, but emphasised it was "completely unrelated" to a report issued by UN investigator Richard Goldstone.
A team of UN investigators led by Goldstone said it found evidence that Israel and Hamas fighters committed war crimes during the conflict.
The two sides deny the accusations.
The UN General Assembly ordered Israel and Hamas to carry out investigations or face possible action from the US Security Council.
Israel has rejected the report and insists it is capable of investigating itself - a claim international human rights groups dismiss.
EU backs Goldstone
On Wednesday, the European Parliament backed the findings of the Goldstone report and urged its 27-member states to monitor Israeli and Palestinian probes into war crimes in Gaza.
It also urged Israel to immediately open its border crossing with the Gaza Strip to alleviate the worsening humanitarian crisis there.
The assembly is the second institution after the United Nations to back the report, with just over 50 per cent of politicians passing the resolution.
But the move was sharply criticised by Israel, which says it has opened 36 criminal investigations into complaints of improper conduct by its troops during the fighting in Gaza, much of which occurred in residential areas.
Last month a senior Israeli field officer involved in the war in Gaza was reprimanded over artillery shelling in a heavily populated area that hit a United Nations compound.
Some 1,400 Palestinians, including hundreds of civilians, and 13 Israelis, were killed in the offensive Israel launched in December 2008.