Intelligence officials hold classified briefings with lawmakers

More than 1,400 people — including 426 children — were killed in last week’s Syrian chemical weapons attack, according to that White House report. 

The unclassified report says that U.S. intelligence assesses “with high confidence” that Syrian President Bashar Assad’s forces carried out the attack in the Damascus suburbs.

The information compiled in the administration report was based on "multiple streams of information" ranging from classified sources and methods to open source intelligence, including social media and other outlets, a second senior White House official said Friday. 

The details of the attack and its aftermath were "fully vetted" by all members of the intelligence community, up to and including Director of National Intelligence James Clapper, the first official said Friday. 

"This is not a new program," the first official said of Assad's chemical weapons program. 

"It is vast, it is extensive and very well run," the official added. 

Secretary of State John KerryJohn Forbes KerryBringing the American election experience to Democratic Republic of the Congo Some Dems sizzle, others see their stock fall on road to 2020 The Hill's 12:30 Report MORE said Friday that the intelligence findings on the Syrian attacks are “as clear as they are compelling.”

“I'm not asking you to take my word for it. Read for yourself, everyone, those listening, all of you, read for yourselves the evidence from thousands of sources, evidence that is already publicly available,” Kerry said.

A day before releasing the detailed report, top U.S. national security officials briefed lawmakers on the attack by the Assad government. 

Along with Kerry, Defense Secretary Chuck HagelCharles (Chuck) Timothy HagelThe Hill's 12:30 Report The Hill's 12:30 Report Billionaires stopping climate change action have a hold on Trump, GOP MORE, National Security Adviser Susan Rice and National Intelligence Director James Clapper were also on Thursday's call with more than two dozen congressional leaders.

But certain lawmakers complained the Thursday briefing was light on details and heavy on White House rhetoric justifying U.S. military action in Syria. 

Obama has repeatedly stated the use of those weapons in the country's civil war would cross a "red line" that triggers a U.S. military response. 

That said, classified and unclassified briefings on the administration's slate of military options regarding Syria have "been going on since day one," the first administration official said. 

Lawmakers began receiving updates on the growing Syrian crisis and possible use of chemical weapons by Assad in August, and they have been kept in the loop on that intelligence since then, the official said. 

"What we have to weigh is how well we present the case" for military action in Syria while preserving the intelligence community's ability to "do our job tomorrow," the official said. "That is the balance we were trying to reach."