Two top Iraqi officials said the attackers were almost certainly members of al Qaida in Iraq or the Nusra Front, one of the most effective groups fighting to topple the regime of Syrian President Bashar Assad. In December, the State Department added the front, known in Arabic as Jabhat al Nusra, to its list of international terrorist organizations, saying it was merely an alias for al Qaida in Iraq. Nusra has been at the forefront of recent rebel gains in Syria.

Iraqi officials said it was still unknown whether the attackers had crossed from Syria into Iraq to launch the attack or were already in the country. The area where the attack took place, Anbar province, was a stronghold of al Qaida in Iraq for much of the time that U.S. troops were based in the country.

The assailants used land mines and light arms to attack an Iraqi military convoy that was escorting Syrian civilians and soldiers, who’d fled to Iraq over the weekend when Nusra fighters seized control of a border crossing. The officials said the assailants had tracked the convoy’s movements, possibly by obtaining military intelligence.