By Barbara Starr

CNN Pentagon Correspondent

CNN has learned the United States has intercepts of conversations among top Syrian military officials discussing the chemical weapons attack after it took place last week, according to a U.S. official.

The intercepts form a key basis for the conclusion that the Syrian regime was behind the attack. But another crucial piece of evidence about what type of chemicals may have been used remains to be determined. Tissue samples and other medical and forensic evidence taken from victims has not yet been fully analyzed, the official said. One source familiar with the latest intelligence said getting that information would provide the strongest case for the use of chemical weapons.

CNN has previously reported Israeli military intelligence provided the United States with intercepts between Syrian military commanders discussing the movement of chemical weapons to the area of the attack before it happened, according to a diplomatic source.

The administration has said it would release a declassified version of its intelligence assessment about last week's attack outside Damascus. White House spokesman Josh Ernest said Thursday the declassified version is not yet complete but should be released by the end of the week.

The intelligence includes an assessment of what level of involvement Syrian President Bashar al-Assad had in last week's chemical attack, but that information remains classified and might not be released to the American public, a senior U.S. official said.

The White House is struggling with how much information to release publicly because it could compromise the sources of intelligence about who ordered the attack, the official said.

President Barack Obama said in an interview Wednesday the U.S. government has "concluded that the Syrian government in fact carried out" the attack - a sentiment echoed by U.S. officials across Washington.

But one question remaining is whether the attack was ordered by al-Assad himself. On that question, the senior U.S. official said, the administration has a "very clear answer" - but declined to say what that answer is - citing the classified nature of the intelligence.

The first U.S. official told CNN the intelligence assessment shows the attack was ordered at the "highest levels" of the regime. That official declined to specifically say whether al-Assad ordered the attack but did not rule it out.

"We are not saying Assad himself had his finger on the button. But he is responsible for the stockpile and any attack like this would have to have been approved at the highest levels," he said. He rejected the notion the evidence of regime involvement was circumstantial.

"We have a very solid case to indicate the regime was behind this."

That official said the evidence shows it was not a "rogue element" of the Syrian regime that carried out the attack, or the Syrian rebels. Another U.S. official echoed the idea that the intelligence assessment shows it was not a rogue element of the regime.

None of these officials spoke on the record due to the sensitive nature of the information.

Publicly, U.S. officials say it is irrelevant whether Assad actually ordered the attack or not, arguing that Assad is responsible for the actions of his regime.

"The commander-in-chief of any military is ultimately responsible for decisions made under their leadership, even if command and control - he's not the one that pushes the button or said, 'Go,' on this," State Department deputy spokeswoman Marie Harf said Wednesday.

On Thursday, the British government released a summary of its intelligence assessment that concluded "it is highly likely that the (Syrian) regime was responsible" for the attack. British intelligence "had high confidence in all of its assessments except in relation to the regime's precise motivation for carrying out an attack of this scale at this time - though intelligence may increase our confidence in the future," the document said.

Separately, the U.S. intelligence community currently is focusing heavily on any real time movements by the Syrian leadership, its forces, or elements of the regime in reaction to the possibility of military action by the U.S., the senior official said.

"We do not want to go blind and deaf," about the location of key elements of the regime. "We want to see what 'looks different,' what 'might be different' and how we stay ahead of the game," the official said.

This becomes crucial to ensuring that all targeting remains up to date and the U.S. has the latest information about the location of chemical weapons sites. The U.S. intelligence community is also trying to assess in detail any possible reactions by al-Assad in advance of a possible U.S. strike, and afterward if one is ordered, including the possibility he will launch more attacks.

"We are not ruling anything out," the official said. There have been reports of some weapons including aircraft being dispersed to avoid a possible U.S. missile strike. The U.S. also believes Assad will continue to move other assets around, but will remain in control of the chemical stockpile, the official said.

The United States also is closely watching Lebanese Hezbollah as well as the Iranians for any signs they could launch proxy attacks against Syria's neighbors, including Israel and Iraq, in retaliation for a potential U.S. strike.