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White House Chief of Staff Denis McDonough told CNN on Sunday that the administration simply does not have “irrefutable, beyond-a-reasonable-doubt evidence” to show Mr. Assad ordered the attack. Instead, Mr. McDonough said the administration relied on the “common-sense test” to more broadly pin the attacks on the “Assad regime.”

Mr. McDonough also suggested the administration was disinterested in the skepticism that such remarks might be triggering. “This is not a court of law,” he said. “And intelligence does not work that way.”

In addressing the nation from the White House Tuesday night, Mr. Obama reiterated a claim that other senior administration officials such as Mr. Kerry have made. “We know the Assad regime was responsible,” Mr. Obama said.

The president used careful language to convey the roots of that conviction. “In the days leading up to Aug. 21, we know that Assad’s chemical weapons personnel prepared for an attack near an area they where they mix sarin gas,” he said. “They distributed gas masks to their troops. Then they fired rockets from a regime-controlled area into 11 neighborhoods that the regime has been trying to wipe clear of opposition forces.”

Mr. Obama’s remarks were a shade broader than the initial case that he and others laid out two weeks ago when the White House circulated an unclassified version of a report that it had titled the “U.S. Government Assessment of the Syrian Government’s Use of Chemical Weapons.”

The classified report remains secret. But the unclassified version made mention of Mr. Assad by name only once, asserting that he “is the ultimate decision maker for the chemical weapons program and members of the program are carefully vetted to ensure security and loyalty.”

The document did not delve directly into the possibility that a rogue Syrian general may have used the chemical weapons without Mr. Assad’s approval. It did, however, seem to hedge around the possibility.

“We have a body of information, including past Syrian practice, that leads us to conclude that regime officials were witting of and directed the attack on August 21,” the report said. “We intercepted communications involving a senior official intimately familiar with the offensive who confirmed that chemical weapons were used by the regime on August 21 and was concerned with the U.N. inspectors obtaining evidence.”

Mr. Kerry has gone perhaps further than any other administration official in his description of the intelligence.

Appearing in London on Monday with British Foreign Secretary William Hague, Mr. Kerry said that “the chemical weapons in Syria we have tracked for some period of time now are controlled in a very tight manner by the Assad regime.”

“It is Bashar al-Assad and Maher al-Assad, his brother, and a general who are the three people who have control over the movement and use of chemical weapons,” Mr. Kerry said. “But under any circumstances, the Assad regime is the Assad regime. And the regime issues orders. And we have high-level regime [members] that have been caught giving these instructions and engaging in these preparations with results going directly to President Assad.”