The probe led by former South African judge Richard Goldstone concluded that "Israel committed actions amounting to war crimes, possibly crimes against humanity," during its Dec. 27-Jan. 18 military operations against Palestinian rocket squads in the Gaza Strip.
In a 575-page report, Goldstone and three other investigators also found evidence "that Palestinian armed groups committed war crimes, as well as possibly crimes against humanity."
Goldstone said the probe, which included 188 interviews, a review of 10,000 documents and 12,000 photos and video, was completed only Tuesday morning, just hours before the hastily called news conference.
"There should be no impunity for international crimes that are committed," he said. "It's very important that justice should be done."
Israel, which refused to cooperate with the investigation, said the Geneva-based U.N. Human Rights Council that ordered it was biased.
The investigators recommended that the U.N. Security Council require Israel to launch its own credible investigation into the conflict within three months. If that is not done, the investigators called on the council to refer the matter for action by the International Criminal Court prosecutor within six months. However, Israel does not accept the court's authority.
The Palestinian group Hamas rules Gaza and has been accused by Israel of using human shields during the conflict, in which almost 1,400 Palestinians were killed - many of them civilians.
"The mandate of the mission and the resolution establishing it prejudged the outcome of any investigation, gave legitimacy to the Hamas terrorist organization and disregarded the deliberate Hamas strategy of using Palestinian civilians as cover for launching terrorist attacks," Israel's Foreign Ministry said.
Goldstone, who is Jewish and has strong ties to Israel, told reporters at U.N. headquarters that "to accuse me of being anti-Israel is ridiculous," anticipating such criticism. He said it was in the interest of both Israelis and Palestinians to establish the truth of what happened in the conflict.
In a joint statement, nine Israeli human rights groups said the findings join a "long series of reports" indicating that Israeli and Hamas violated the laws of war. It called on the Israeli government to conduct an "independent and impartial investigation."
"The groups expect the government of Israel to respond to the substance of the report's findings and to desist from its current policy of casting doubt upon the credibility of anyone who does not adhere to the establishment's narrative," it said.
In a preliminary investigation earlier this year, the army cleared itself of any systematic wrongdoing during the war and said any rights violations were isolated incidents. Since then, it has opened a series of separate investigations into the conduct of individual soldiers.
"Notwithstanding its reservations, Israel will read the report carefully," the Foreign Ministry said Tuesday of the U.N. report, noting that the military has examined more than 100 allegations regarding the conduct of its forces during the Gaza operation, resulting in 23 criminal investigations.
Hamas officials were not immediately available for comment.
The report said that Israel's attacks in the Zeitoun neighborhood of Gaza City, including the shelling of a house where soldiers had forced Palestinian civilians to assemble, amounted to war crimes.
It found seven incidents in which civilians were shot while leaving their homes trying to run for safety, waving white flags and sometimes even following Israeli instructions, as well as the targeting of a mosque at prayer time, killing 15 people, were also war crimes.
A "direct and intentional attack" on the Al Quds Hospital and an adjacent ambulance depot in Gaza City "may constitute war crimes," the report said.
Several Palestinians told the mission they were used as human shields by the Israeli forces, the report said, noting the case of Majdi Abd Rabbo, a 39-year-old intelligence officer of the Palestinian authority who was forced to walk ahead of the troops as they searched his and his neighbor's house. Rabbo was forced to undress down to his underwear in front of the soldiers and his sons had to strip naked, the report said.
On the Palestinian side, the report found that armed groups firing rockets into southern Israel from Gaza failed to distinguish between military targets and the civilian population.
"Where there is no intended military target and the rockets and mortars are launched into civilian areas, they constitute a deliberate attack against the civilian population," the report said. "These actions would constitute war crimes and may amount to crimes against humanity."
Investigators called on Israel to immediately allow people and goods across borders "for the recovery and reconstruction of housing and essential services and for the resumption of meaningful economic activity in the Gaza Strip."
They also recommended that Israel ease up on fishing restrictions within 20 nautical miles from shore and allow farming to resume within the Gaza Strip "including within areas in the vicinity of the borders with Israel."