U.S. soldiers found mass graves north of Baghdad next to a torture center where chains were attached to blood-spattered walls and a metal bed frame was still connected to an electrical shock system, the military said Thursday.

Separately, at least 13 Iraqis were killed when a suicide bomber targeted a group of people who had gathered around U.S. soldiers handing out holiday gifts, local authorities said. It was not immediately known if any soldiers were killed or injured.

The grisly discoveries of the mass graves and torture center near Muqdadiyah, about 60 miles north of Baghdad, came during a Dec. 8-11 operation that also saw multiple battles between American troops and militants. The military said it killed 24 insurgents and detained 37 others during the operation.

The torture center, which the military said it thinks was run by al-Qaida in Iraq, was found based on tips from local Iraqis. Graves containing 26 bodies were found nearby.

"We discovered several (weapons) caches, a torture facility that had chains, a bed — an iron bed that was still connected to a battery — knives and swords that were still covered in blood as we went in to go after the terrorists in that area," said Army Maj. Gen. Mark P. Hertling, the top U.S. commander in northern Iraq.

Soldiers found a total of nine caches containing a surface-to-air missile launcher, sniper rifles, 130 pounds of homemade explosives and numerous mortar tubes and rounds, among other weapons.

Despite a nationwide decrease in violence of nearly 60 percent, Diyala province, where the torture center was found, is still turbulent — largely because the summer influx of U.S. troops in Baghdad, a freeze on activities by the Mahdi Army militia and the rise of Sunni anti-al-Qaida "awakening" groups have pushed militants into the area.

"Yes, there are still some very bad things going on in that province," Hertling said. "We are slower in coming around because ... some of the extremists have been pushed east from Anbar province as they've seen the awakening movement there and north from Baghdad as the surge operations took place there."

Hertling did point out, however, that the number of roadside bombings against coalition and Iraqi troops in the area had decreased between 40 and 50 percent since the summer. He said there were 849 such attacks in November as compared to 1,698 in June.

But he also warned that al-Qaida in Iraq was still capable of massive violence.

"You know, there's going to be continued spectacular attacks," Hertling said. "We're trying, along with the Iraq Army, to protect all the infrastructure of Iraq. These people who are fighting us, who are fighting the Iraqi people, continue to just destroy with no intent to contribute to what Iraq is trying to be."

In Baghdad, shops were closed and the streets were empty as the Eid al-Adha holiday was observed.

Eid al-Adha is a holy celebration for Muslims, commemorating the prophet Abraham's willingness to sacrifice his son for God. According to Muslim tradition, after Abraham expresses his willingness, God sends the prophet two sheep instead for slaughter.

Violence this week has been down across Iraq — even in comparison to the recent drops in attacks — perhaps as a result of the holiday. On Wednesday, only one body was found in Baghdad and there was just one reported killing.

However, 13 civilians were killed Thursday east of Baqouba, 35 miles northeast of Baghdad, when a man wearing an explosives vest blew himself up amid a crowd that had gathered around U.S. soldiers handing out holiday gifts, a local policeman said on condition of anonymity as he was not authorized to release the information.

It was not clear if the attack near Kanaan, a Shiite-dominated town about 13 miles east of Baqouba, killed or wounded any of the soldiers. At least 18 people were hurt in the attack.

Separately, the U.S. military said that its preliminary investigation into a Dec. 17 incident in which a Marine killed an Iraqi policeman as they manned a joint security station north of Ramadi showed both men suffered cuts during a fight. It was not clear what sparked the altercation.

The military said the Marine, who was not identified and was treated at a hospital and released after the fight, was not yet facing charges, but that the investigation was ongoing.