BAGHDAD (AP) — Militants from the Islamic State group carried out a mass killing of hundreds of Iraqi soldiers captured when the extremists overran a military base north of Baghdad in June, a leading international watchdog said Wednesday.

The incident at Camp Speicher, an air base that previously served as a U.S. military facility, was one of the worst atrocities perpetrated by the Islamic State group in its lightning offensive that seized large swaths of northern and western Iraq.

According to Human Rights Watch, new evidence indicates the Islamic State fighters killed between 560 and 770 men captured at Camp Speicher, near the city of Tikrit — a figure several times higher than what was initially reported.

“These are horrific and massive abuses, atrocities by the Islamic State, and on a scale that clearly rises to the crimes against humanity,” Fred Abrahams, special HRW adviser, told reporters in the northern city of Irbil on Wednesday.

The al-Qaida-breakaway claimed in mid-June that it had “executed” about 1,700 soldiers and military personnel from Camp Speicher. The group also posted graphic photos that appeared to show its gunmen massacring scores of Iraqi soldiers after loading the captives onto flatbed trucks and then forcing them to lie face-down in a shallow ditch, their arms tied behind their backs.

The grisly images, meant to sap the morale of Iraqi security forces, and the number of slain troops could not be confirmed at the time. Human Rights Watch said in late June that analysis of photos and satellite images showed that between 160 and 190 men were killed in at least two locations between June 11 and 14.

After the incident, the soldiers were listed as missing, prompting their families to stage demonstrations in Baghdad in an effort to pressure authorities for word on their sons’ fate. On Tuesday, dozens of angry family members stormed into the parliament in Baghdad’s fortified Green Zone after scuffling with security guards, causing commotion and arguing with lawmakers. They also forced the speaker to call a session for Wednesday on the missing soldiers.

The Human Rights Watch statement said the revised figure for the slain soldiers was based on analysis of new satellite imagery, militant videos and a survivor’s account that confirmed the existence of three more “mass execution sites.” The number of victims may well be even higher as more evidence emerges, the New York-based watchdog said.

“Another piece of this gruesome puzzle has come into place, with many more executions now confirmed,” said Peter Bouckaert, emergencies director at Human Rights Watch. “The barbarity of the Islamic State violates the law and grossly offends the conscience.”

Also Wednesday, the U.N. envoy in Iraq called for a public and independent investigation by Iraqi authorities into the fate of the missing soldiers and the recovery of the remains of those killed.

The investigation is needed “to locate and identify the remains of any who may have been killed, and to undertake all efforts to secure the release of any who may remain in captivity,” Nickolay Mladenov said.

In his weekly address to the nation, outgoing Prime Minister Nouri al-Maliki said Wednesday that a number of “perpetrators” of Camp Speicher atrocities have been arrested or killed and that “security forces were pursuing” others. Al-Maliki did not elaborate.

The onslaught by the Islamic State group has stunned Iraq’s security forces and the military, which melted away and withdrew as the extremists advanced and captured key cities and towns. The militants also targeted Iraq’s indigenous religious minorities, including Christians and followers of the ancient Yazidi faith, forcing tens of thousands from their homes.

Since then, the Islamic State group has carved out a self-styled caliphate in the large area straddling the Iraqi-Syrian border that it now controls.

In early August, the United States launched airstrikes on the militant group in Iraq, in an effort to help Iraqi forces fight back against the growing militant threat.

Also this week, the United Nations’ top human rights body approved a request by Iraq to open an investigation into suspected crimes committed by the Islamic State group against civilians. Its aim would be to provide the Human Rights Council with evidence on atrocities committed in Iraq, which could be used as part of any international war crimes prosecution.

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Associated Press writer Sameer N. Yacoub in Baghdad contributed to this report.