Nusra first came to the world’s attention nearly a year ago when a car bomb exploded in Damascus, killing at least 44 people and wounding another 160. That Dec. 23 bombing was followed by another on Jan. 6 that killed at least 26 people and injured dozens of others. At the time, Damascus largely had been free of the violence that was growing elsewhere in Syria.

Opposition leaders at first disavowed the blasts and blamed the Assad regime for them. But U.S. officials told McClatchy in early February that they believed the explosions were the work of al Qaida in Iraq and that al Qaida’s leader, Ayman al Zawahiri, had authorized his followers in Iraq to move into Syria.

In the ensuing months, suicide bombings became an increasing tactic of the anti-Assad forces, and Nusra fighting units began to appear in key battles around the country, particularly after rebels launched offensives in July in Aleppo, Syria’s commercial hub, and Damascus.

U.S. officials on Monday declined to detail the evidence that led them to designate Nusra as another name for al Qaida in Iraq or explain why it took so many months for them to reach that conclusion. But Nusra fighters have told a McClatchy reporter and other researchers that Iraqis are believed to be among Nusra’s top commanders and fighters, working in conjunction with Syrian foot soldiers. Many Syrian fighters have acknowledged in interviews that they fought against U.S. forces in Iraq.

The veteran militants seem to have learned some lessons from missteps in Iraq that cost them public support. Instead of intimidating Syrians into accepting their literalist views, they’ve provided basic services. In Iraq, al Qaida-allied cells often worked in isolation from other types of insurgents and wore out their welcome in host communities by forcing locals to adopt their rigid interpretation of Islam, even banning smoking in some regions.

Zelin noted that Syria was a major conduit for men, weapons and cash supplies to al Qaida in Iraq insurgents who fought the U.S.-led military occupation of Iraq.

“There was already a network of individuals and an infrastructure in Syria even before the uprising began,” he said. “So this is a legacy remnant of al Qaida in Iraq.”

After McClatchy reported last week that the State Department was poised to designate the Nusra Front a terrorist group, other Syrian rebels expressed solidarity with the group and dismissed the U.S. move as a ploy to cover how little the Obama administration has helped the rebels militarily. The Obama administration supplies only “nonlethal” assistance such as medical and communications help to the Syrian opposition, though it quietly supports some of its Persian Gulf allies in sending arms.

“We are all Jabhat al Nusra,” read a joint statement in Arabic from 29 Syrian local committees and militias that reportedly have sworn solidarity to the Nusra Front.

The Reuters news agency and Arabic media also reported on the formation of the new Islamist-dominated rebel council, which was set up in opposition to the Turkey-based Syrian rebel command that’s affiliated with a new opposition council that hopes to win U.S. recognition as a government in waiting.

That kind of rift, which underscores the battles over legitimacy among Syrian opposition leaders, is exactly what the United States was hoping to avoid by trying to isolate extremists and boost the relatively moderate forces that have vowed to restore Syrian ties to the international community with a post-Assad nation that’s pluralistic and democratic.

McClatchy special correspondent David Enders contributed to this report from Beirut.