Story highlights There was "no war crime committed" in an incident that killed 21, the Israeli military says

The 21 members of the al-Samouni family were killed during an Israeli offensive

Israeli human rights group B'Tsalem says "the army should not be investigating itself"

Israeli military investigatorshave ordered an end to a probe into the deaths of 21 members of a Palestinian family during the 2009 military offensive on Gaza, saying there was "no war crime committed."

The al-Samouni family members and others died when a shell hit a Palestinian residential building. It was one of the most controversial incidents of the offensive.

One of the surviving family members, Maysa al-Samouni, told CNN in January 2009 that her family and other families were ordered by Israeli soldiers to move from one house to another until dozens were cramped in one building, which then came under fire.

"The first bomb wounded three and killed one," she said. "We called for ambulances, and then another bomb hit and lots of people -- at least 30 -- were killed, including my husband, mother-in-law, and uncle."

Other witnesses CNN spoke to at the time said Israeli soldiers were in the area and it wasn't until the following day medics were allowed in to evacuate the wounded and the dead.

The Israeli human rights group B'Tsalem and the Palestinian Center for Human Rights had filed separate complaints alleging war crimes and demanding a proper, transparent and independent investigation into the incident.

In a letter to B'Tsalem on Tuesday, the Military Advocate General said that after investigating, "we found the accusations are groundless."

The investigators "found that none of the involved soldiers or officers acted in a negligent manner," and "ordered the closure of the file," the letter said. But it added that the military is making changes to "ensure that such events will not happen again."

Sarit Michaeli, spokeswoman for B'Tsalem, told CNN that the organization is seeking details of the investigation.

"The army should not be investigating itself," Michaeli said. "There should be an internal Israeli body external to that of the military looking into these kinds of suspicions."

She said the group wants to see "if this investigation adhered to international standard applied to this type of the investigation. One of the standards of the investigation (is) it has to be timely. We can say already that it did not adhere to that standard because the investigation was opened very late."

Israel launched the 23-day Operation Cast Lead in late December 2008, resulting in the reported deaths of more than 1,400 Palestinians and 13 Israelis. The offensive was aimed at curbing what Israel described as the firing of rockets into southern Israel by militants in Gaza.

United Nations' Human Rights Council commissioned a team, led by jurist Richard Goldstone, to look into allegations of war crimes during the operation, and it issued a detailed report concluding that both Israel and Hamas, the Palestinian group controlling Gaza, had committed crimes against humanity.

Statements from witnesses also were collected by the Israeli and Palestinian human rights groups.

B'Tsalem said one witness reported that "more than 100 residents of the Al-Zeitoun neighbourhood in Gaza were ordered by the Israeli military, who raided the neighborhood then, to stay in one house."

"A day later the house was shelled by the Israeli military, which resulted in the collapse of the entire house and the immediate killing of more than 20 members of the al-Samouni family," B'Tsalem said.