GAZA — U.N. investigators said on Monday Israel and Palestinian militant groups committed grave abuses of international humanitarian law during the 2014 Gaza conflict that may amount to war crimes.

They called on all sides to cooperate with the International Criminal Court (ICC), which has opened a separate preliminary investigation. The Palestinian Authority made its first formal submission to the Hague-based court this week.

“The most that we can hope for out of this long and arduous process of inquiry is that we will push the ball of justice a little further down the field,” Mary McGowan Davis, chairwoman of the U.N. commission of inquiry, said at a news conference.

A ceasefire last August ended 50 days of fighting between Gaza militants and Israel, in which health officials said more than 2,100 Palestinians, mostly civilians, were killed. Israel put the number of its dead at 67 soldiers and six civilians.

Israeli air strikes and shelling hammered the densely populated Gaza Strip, dominated by the Islamist Hamas movement, causing widespread destruction of homes and schools.

Hamas and other militant groups launched thousands of rockets and mortar bombs out of the enclave into Israel.

“The (U.N.) commission was able to gather substantial information pointing to serious violations of international humanitarian law and international human rights law by Israel and by Palestinian armed groups. In some cases, these violations may amount to war crimes,” the independent investigators said in a report issued Monday, following a year-long inquiry.

The investigators urged Israel to carry out credible domestic investigations of senior political and military officials, if crimes were substantiated.

They called on Israel to explain its “targeting decisions” to allow independent assessment of its attacks on Gaza Strip which they said had killed 1,462 civilians.

“The commission is concerned that impunity prevails across the board for violations of international humanitarian law and international human rights law allegedly committed by Israeli forces…,” it said.

The report said Israel should break with its “recent lamentable track record in holding wrongdoers responsible.”

Israel disputed the findings, saying its forces acted “according to the highest international standards.”

The independent investigators also condemned the executions of 21 alleged Palestinian “collaborators” with Israel by militants in Gaza, saying the killings appeared to constitute war crimes.