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(CNN) - Pulitzer Prize-winning journalist Seymour Hersh published an explosive, and hotly-disputed piece Sunday, alleging the Obama administration cherry-picked the intelligence it had about who used sarin gas to kill hundreds of Syrian civilians in August.
"We know the Assad regime was responsible. In the days leading up to August 21, we know that Assad's chemical weapons personnel prepared for an attack near an area where they mix sarin gas. 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," President Barack Obama said in an address to the nation in September.
"If we fail to act, the Assad regime will see no reason to stop using chemical weapons," Obama said.
However, Hersh says that it is not so crystal clear that Syrian President Bashar al-Assad was the one who used the chemical weapons.
"I'm not saying I know. But I don't think our administration knew either," Hersh said in an interview with CNN's "The Lead with Jake Tapper."
CNN has not verified any of Hersh's reporting.
"We know, and we've known since spring, that the most radical jihadi opposition group, rebel group al-Nusra ... had access to sarin. There was a lot of intelligence reporting about it, the White House can say anything they want about it, but that is a fact," said Hersh.
When incidents of chemical attacks in Syria came to light, "what they didn't do was consider the possibility that al-Nusra could be one of the people involved, they went right away to the notion that it had to be Bashar," said Hersh. "And in fact, they didn't have much of a case."
The White House strongly disputes Hersh's report.
"The intelligence clearly indicated that the Assad regime and only the Assad regime could have been responsible for the 21 August chemical weapons attack. There's no evidence to support Mr. Hersh's claims to the contrary and the suggestion that there was an effort to suppress intelligence is simply false," Director of National Intelligence spokesman Shawn Turner told CNN in a statement.
"As of December 9, 2013, no U.S. intelligence agency assesses that the front has succeeded in developing a capability to manufacture sarin," Turner told CNN.
"That's contradictory to the intelligence they have, and that happens," said Hersh. "This is an embarrassing thing for a president of the United States."
For more of our interview with Seymour Hersh, watch the video above.