Baghdad ? A suicide bomber struck the funeral of two Sunni tribesmen who joined forces against al-Qaida in Iraq, killing at least 50 people Thursday and reinforcing fears that insurgents are hitting back after American-led crackdowns.

The sudden spike in bloodshed this week adds to the other worries now piling up in Iraq: violent rivalries among Shiites and persistent cracks in the Iraqi security forces.

Violence across the country has declined since seven months ago, including dramatic suicide bombings like Thursday’s funeral attack. American officials credit the change to the U.S. troop buildup and the rise of Sunni tribal groups known as Awakening Councils that have turned against al-Qaida-linked militants. A truce called last year by anti-U.S. Shiite cleric Muqtada al-Sadr has also helped.

But the new bloodshed highlights how fragile those gains are.

Thursday’s attack happened in the town of Albu Mohammad, about 90 miles north of Baghdad. A suicide bomber dressed in traditional Arab robes passed un-searched by guards into a tent of mourners. The occasion was a funeral for two brothers who belonged to the local Awakening Council and who were killed in an attack a day earlier.

The bomber detonated explosives strapped to his body, killing at least 50 people and wounding dozens more, said police in the nearby city of Kirkuk.

“I first heard a thunderous explosion and when I turned my eyes to the tent I saw fire and smoke coming out,” said Sheik Omar al-Azawi, an Awakening Council member who arrived at the funeral just before the blast. “Panicked people were jumping and running on all sides.”