Bassam Khabieh / Reuters Free Syrian Army fighters escort a convoy of U.N. vehicles carrying a team of chemical weapons experts to one of the sites of an alleged chemical weapons attack in Damascus' suburb of Zamalka on Wednesday.

The United States is looking into whether a failed assassination attempt against Syrian President Bashar al-Assad may have motivated last week’s deadly chemical weapons attack against rebel-held enclaves outside Damascus.

U.S. officials said Tuesday that the possibility that an Aug. 8 assassination attempt on Assad prompted him or a senior official in his regime to order the use of chemical weapons last week against towns in the suburban district of Ghouta is a “working theory that analysts are looking at.”

“It’s at the very least plausible,” said one official, who like the others spoke on condition of anonymity.

Salim Idris, commander of the Free Syrian Army, said sources in Assad’s inner circle tell him that’s exactly what happened.

He said the attack on the rebel enclaves around the capital on Aug. 21 was ordered as pressure mounted on Assad – even from his own supporters – for a reprisal to the Aug. 8 attack, in which rebels fired rockets at his motorcade as it rolled through the Malki neighborhood of Damascus, headed to a local mosque for prayers marking the end of Ramadan.

Assad is not believed to have been in the motorcade at the time of the rocket attack, but the brazen assault on the president’s own doorstep had a chilling effect on Assad and senior military officials, according to Idris.

Idris also indicated that pressure also has been growing on Assad to respond to a series of rebel advances.

One of the biggest gains for the rebels was the capture in July of large stockpiles of weapons from government depots near Qalamoun north of the capital, including 600 Concourse anti-tank rockets, according to Idris.

Brigadier General Salim Idris, the commander of the Free Syrian Army, tells NBC's Richard Engel the Syrian government used chemical weapons "more than four times" against civilians, dropping them from planes.

An equally important reason for taking drastic action may have been recent fighting in what Syrians call the “Sahel,” the coastline around Latakiya that is the stronghold of Assad’s regime and his religious base of support, Idris said.

The Assad family is Alawite, an offshoot of Shiite Islam. The rebels, like the population in general, are overwhelmingly Sunni Muslims. Since the war began, the Sahel has been a stable rock of Assad support -- safe and seemingly immune to the war. But fighting has intensified there recently in government protected Alawite villages. Rebels even captured the Alawite religious leader Sheikh Badr Grazel, and according to some reports have killed him. The fighting in the Sahel shook the confidence of some in the Alawite community who complained that Assad had lost his ability to protect them, Idris said.

U.S. officials confirmed that the Syrian rebels have made tactical advances in recent weeks -- including seizing some checkpoints near Latakiya last week – that have put them on a more even footing with pro-Assad forces. They did not confirm the seizure of the Syrian weapons near Qalamoun, however.

According to accounts provided to NBC News by more than a dozen witnesses, medical personal, victims and rebel leaders, the attack on Aug. 21 did not initially trigger undue alarm among war-weary residents of Ghouta and other rebel-held suburbs and villages outside Damascus. Many barely noticed the first explosions, assuming it was just more shelling in the 2½-year-old war. There wasn’t any place for people to run and hide anyway. The villages and neighborhoods were surrounded by Syrian forces, blockaded like castles in a medieval war. Most people just rolled over and went back to sleep, waiting for the shelling to subside.

But this attack was different. At least 29 rockets were fired at Ghouta over a two-hour period beginning at 2:45 a.m., according to these accounts. It’s unknown how many of the rockets were armed with poison gas, or what was in the warheads.

Rebels say the rockets were fired from at least four locations, including the Mezzeh military airport and the Qassioun plateau on the edge of Damascus, where a network of military bases is located. The plateau was previously attacked by Israel, suspecting Hezbollah was moving weapons in the area.

Idris and other rebel commanders say the area where the chemical weapons were launched in Qassioun was under the control of the Syria’s 4th Armored Division, commanded by President Assad’s brother Maher al-Assad,. Other units involved, according to Idris, were the 155th and 127th brigades, and Syria’s Air Force intelligence service. Idris said the two main planners of the attacks were Gen. Taher Hamid Kandil and Gen. Grassan Abass, both of whom command artillery and surface to surface missiles. U.S. officials did not dispute the claims.

The accounts from witnesses and others in Ghouta indicated the timing of the early morning attack was ideal for causing maximum casualties.

The August sun hadn’t yet risen and stirred up wind that could blow the chemical weapons off target. Nor was there direct sunlight, which could break down Sarin gas if it was present.

Very quickly, people began to choke. Some convulsed. Children died the quickest, some still in their beds and cribs. A few survivors said they noticed a strange smell. One man said it was like insecticide. Rebels lit tires on fire on the streets, thinking the black smoke would somehow offset the chemical gas. It only added to the confusion and made more people choke.

The wounded began rushing into hospitals. First dozens. Then hundreds. Then thousands.

Volunteers doused them with water and vinegar, trying to wash off whatever was causing them to writhe in pain.

Medical staff started injecting the injured with atropine, a treatment for nerve agent. They kept pouring water on the victims, but it began infecting medical personnel when it pooled on the floor and bathed them in traces of the mystery chemical agent.

Syrian Ambassador to United Nations, Bashar Ja'afari, comments on the reports of chemical weapons use by President Assad, saying "the Syrian government is totally innocent of these accusations."

Bodies were brought to separate rooms. There were so many – hundreds, according to witnesses – that volunteers wrote numbers on pieces of tape, and stuck them to their foreheads.

Syria has denied any involvement in the attacks, and on Tuesday Foreign Minister Walid Muallem said no government would ever consider using weapons of mass destruction against its own people.

"They said the Syrian forces were the ones who carried out this attack. I deny this utterly and completely to Kerry," he said, referring to a statement Monday by U.S. Secretary of State John Kerry declaring that the Syrian government’s involvement in the attacks was “undeniable.”

But the Obama administration was not backing down.

“There is no doubt here that chemical weapons were used on a massive scale on Aug. 21 outside of Damascus,” White House spokesman Jay Carney said. “There is also very little doubt, and should be no doubt for anyone who approaches this logically, that the Syrian regime is responsible.”

Richard Engel is NBC News' chief foreign corrrespondent; Robert Windrem is a senior investigative producer.

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