Social media helped American intelligence officials determine that Syrian President Bashar Assad's regime carried out a deadly nerve gas attack on its citizens earlier this month, top White House aides said Friday, but some members of Congress remain wary of the U.S. launching a retaliatory strike.

The Obama administration, working to build a case for a potential military attack against the Assad regime, released a four-page, unclassified assessment that concluded that the Syrian government used chemical weapons in an attack on twelve neighborhoods in the Damascus suburbs, killing 1,429 people, including at least 426 children.

The assessment outlines the case against Assad, saying the August 21 attack began in the early morning hours when the regime began firing rockets and artillery at the 12 neighborhoods, all of them either controlled by the Assad opposition or contested by the two sides.

Intelligence on the ground and satellite images prove the rockets were fired into the neighborhoods from Assad-controlled areas, White House aides said Friday.

Reports of nerve gas in the area began surfacing on social media around 2:30 a.m. local times, shortly after the gas struck.

“Within the next four hours there were thousands of social media reports on this attack from at least 12 different locations in the Damascus area,” the report indicates. “Multiple accounts described chemical-filled rockets impacting opposition-controlled areas.”

White House aides said multiple sources — including international and Syrian medical personnel who treated victims — to confirm that Assad was behind the attack, not just social media.

The aides declined to provide a specific link between the chemical attacks and Assad, however, and some lawmakers, citing a lack of clear-cut evidence, do not want Obama ordering a military strike, at least not yet.

Sen. Joe Manchin, D-W.Va., said the information provided by the administration so far does not justify a military strike.

“I implore the president to immediately call Congress back to Washington and engage in a serious debate on this issue before any military action is taken,” Manchin said.

Senate Armed Services Committee Chairman Carl Levin, D-Mich., said Friday that Obama shouldn't act without international support.

“The United States should not undertake a kinetic strike before the U.N. inspectors complete their work, and that the impact of such a strike would be weakened if it does not have the participation and support of a large number of nations, including Arab nations,” Levin said.

There are other lawmakers, however, who believe the president should not only act but use enough force to overthrow Assad, who have been responsible for thousands of deaths.

Sens. Lindsey Graham, R-S.C., and John McCain, R-Ariz., released a statement Friday warning Obama against taking only superficial action, as Obama suggested he would do in a PBS interview in which he advocated a “shot across the bow” in response to the chemical attacks.

“The purpose of military action in Syria should not be to help the president save face,” Graham and McCain said. “It should not be merely cosmetic. Instead, the goal of military action should be to shift the balance of power on the battlefield against Assad and his forces.”

White House aides who briefed reporters said Obama has not decided how he would act, but insisted the evidence that Assad was behind the attacks is indisputable.

“The United States made clear some time ago that President Assad is responsible for the use of chemical weapons by his regime and there can be no doubt about that,” one of the aides told reporters. “He is the responsible party.”

The assessment also pointed to more than 100 videos showing dead and injured people who appear to be suffering the effects of nerve gas exposure, including foaming from the nose and mouth.

The report says “Syrian chemical weapons personnel” with gas masks were seen preparing for the attacks on Aug. 21.

White House aides said member of Congress are receiving additional classified information linking the chemical attack to the Assad regime.

House Speaker John Boehner, R-Ohio, signaled Friday that he is not prepared to back a military strike. He sent list of questions about Obama’s strategy on Syria earlier this week.

“As we have said, if the president believes this information makes a military response imperative, it is his responsibility to explain to Congress and the American people the objectives, strategy, and legal basis for any potential action,” Boehner spokesman Brendan Buck said. “We — and the American people — look forward to more answers from the White House.”