Updated

The momentum for a U.S. strike in Syria faces a fresh challenge on Capitol Hill as new evidence surfaces that could complicate the Obama administration’s case.

A new video smuggled out of Syria purportedly showing rebel fighters executing seven unarmed government soldiers is raising questions over whether the U.S. should intervene militarily in the ongoing Syrian conflict and the unintended consequences of instability in the region.

The amateur footage, obtained by The New York Times, shows Syrian rebels holding firearms and standing over seven shirtless, kneeling male prisoners with their hands tied behind their backs and their heads pressed to the ground. On the video a man, called “Uncle,” and presumed to be a rebel commander, starts to recite a poem.

“For fifty years, they are companions to corruption,” the man said, according to a translation by the Times. “We swear to the Lord of the Throne, that this is our oath: We will take revenge.”

When the poem ends, the commander fires a bullet into the back of the first prisoner’s head. One by one the sound of the guns going off can be heard and in the end, all of the kneeling men have been shot dead.

The clip displays the brutality and blurred lines of the growing civil war. It also highlights the growing challenge the White House has in persuading Congress to authorize military action and some of the consequences of American intervention.

Some lawmakers have already warned that the Obama administration, should it launch missile strikes on the Assad regime, would be aiding opposition groups that are not much better.

Liberal Rep. Alan Grayson, D-Fla., on Thursday said the White House is “not telling all sides.” Sen. Ted Cruz, R-Texas, had alleged that the U.S. would effectively be “Al Qaeda’s air force” – though he’s been scolded by his colleagues for the remark.

One-time supporters of limited U.S. involvement like Democratic Sen. Joe Manchin of West Virginia backed off Thursday on moving forward with military action, though he did not cite the video.

“Given the case that has been presented to me, I believe that a military strike against Syria at this time is the wrong course of action,” Manchin said in press statement. “I cannot support the Senate Foreign Relations Committee’s resolution and will be working with my colleague and the administration to develop other options.”

In an Aug. 30 op-ed piece for The Wall Street Journal, Elizabeth O’Bagy, an expert at the Institute for the Study of War, championed American involvement and argued that “contrary to many media accounts, the war in Syria is not being waged entirely, or even predominantly, by dangerous Islamists and al-Qaida die-hards.”

On Wednesday, Secretary of State John Kerry encouraged members of the House of Representatives to read O’Bagy’s WSJ piece and cited her as an expert. O’Bagy has also appeared on several Fox News shows.

However, on Thursday her credibility came under fire after it was revealed that she was also the humanitarian aid coordinator for a Syrian opposition lobbying group – the Syrian Emergency Task Force- with links to the Free Syria Army.

Other staunch supporters of military action like Sen. John McCain, R-Ariz., who have been vocal from the start about a strike are now taking their arguments to their constituents.

McCain is scheduled to hold a town hall meeting in Arizona Friday to discuss military action before he makes any more public declarations of support or opposition.

In the two-and-a-half years since Syria’s civil war began, much of the political and geographical landscape has turned into a layered landmine of criminal activity. In the absence of western aid, rebel soldiers have had to rely on support from some extremist groups linked to Al Qaeda and other terrorist organizations. The concern is that American military action could inadvertently strengthen criminals and extremists who have spoken out against the United States, its people and policies.

P.J. Crowley, a former State Department spokesman, warned that one of the biggest unintended consequences of involvement in Syria could be a destabilizing countries surrounding Syria.

Rep. Buck McKeon, R-Calif., told Fox News Thursday that he had reservations about American intervention in Syria.

“We’ve seen a lot of intelligence and a lot of push and I think it’s going to be a real tough fight,” he said.

For McKeon, the bigger problem is funding an American-led operation.

“The elephant in the room is the sequestration that is hanging over the heads of our military,” he said.