DigitalGlobe Satellite photo shows Syria's Furqlus weapons depot near the city of Homs. The depot is one of five permanent locations where the regime of Bashar Assad stores chemical weapons, according to arms control experts and U.S. officials.

As the Obama administration seeks to rally support for a powerful missile attack on key Syrian targets, much about Damascus’ chemical warfare program remains shrouded in secrecy.

Experts agree that the program is the most advanced in the Third World, and that the Syrian government has used the poisonous arms against its own people "multiple times" in recent years.

Based on recent interviews with U.S. officials, allied intelligence officials and arms control experts, here’s what is known – and not known – about Syria’s chemical weapons arsenal and the looming showdown over what the U.S. says is its most recent attack:

The arsenal

The size of Syria’s chemical weapons arsenal is not known with any precision, though the French government has estimated it to be more than 1,000 tons.

The lack of certainty poses a problem, according to intelligence and arms control experts. “No one but the Syrians knows the inventory, and if the rebels overrun one of these depots, there are worries about the physical control of the weapons," said one U.S. official, who like the others spoke on condition of anonymity.

The experts say that Damascus’ program is distinguished by its size, diversity, quality and reliability. President Bashar Assad’s regime is believed to possess sarin, VX (a persistent form of sarin that could render a city uninhabitable “for some days,” according to the CIA), tabun (another older nerve agent) plus blistering agents like mustard, phosgene and hydrogen cyanide. In addition, it is believed to have large stores of “precursor chemicals” that it could use to create more of the toxic agents.

The weapons are stored in five major locations -- near the cities of Latakia, Palmyra, Homs and Hama, all in the north and central part of the country, and at al-Safir, near the Turkish border.

However, weapons have been moved around the country over the last year for operational reasons, a process that has “accelerated quite a bit” as the threat of a retaliatory strike by the U.S. has increased, according to Pentagon officials. At the same time, U.S. intelligence has been tracking the movements -- not because the U.S. intends to target the chemical weapons, but in an effort to determine if the Syrians may be preparing for new attacks against the rebels.

Weapons systems

Syria has many ways of using chemical weapons to attack remote locations, including a few dozen SS-21 ballistic missiles with a maximum range of 72 miles; 200 Scud-Bs, with a maximum range of 180 miles; and 60 to 120 Scud-Cs, with a maximum range of 300 miles. All these missiles are mobile, enabling Assad and his generals to quickly move or hide them, according U.S. intelligence officials.

Syrian fighter-bombers also can carry bombshells filled with chemicals. Even simple artillery shells can be loaded with chemical weapons and fired at targets within a relatively short range, according to U.S. experts.

Rationale

Syria's chemical weapons arsenal is so large and diverse because it considers it a strategic deterrent to Israel's nuclear weapons. Syria is one of only seven nations in the world that has not ratified the 1992 Chemical Weapons Convention, the arms-control agreement that outlaws the production, stockpiling and use of such weapons. Only Syria, North Korea, Egypt, South Sudan and Angola have not signed the accord for a variety of reasons. Israel and Myanmar have signed but not ratified the convention.

History of use

The U.S. reported last week that Syria has used chemical weapons "on a small scale against the opposition multiple times in the last year.” The British government places the number of attacks at 14 since 2012. Chemical weapons also were used under Assad's father, Hafez, more than 20 years ago in Hama during a Muslim Brotherhood-inspired rebellion. According to a November 1990 Senate Foreign Relations Committee memo, Syrian army units went to every house suspected of hiding insurgents and pumped in cyanide gas, killing as many as 20,000 occupants. Later, the government broadcast a report saying security forces had taken fierce reprisals against the Brotherhood and its sympathizers, "which stopped them from breathing.”

Attack on Ghouta

In its unclassified report on the Aug. 21 attacks on rebel-held enclaves in the Ghouta district outside Damascus, the White House said that preparations began several days before the pre-dawn bombardment.

Citizens crowded in tight spaces to see their congressional representatives and voiced their displeasure with the proposed Syria plan. NBC's Kelly O'Donnell reports.

"Syrian chemical weapons personnel were operating in the Damascus suburb of ‘Adra from Sunday, Aug. 18, until early in the morning on Wednesday, Aug. 21, near an area that the regime uses to mix chemical weapons, including sarin,” it said. “On Aug. 21, a Syrian regime element prepared for a chemical weapons attack in the Damascus area, including through the utilization of gas masks."

Secretary of State John Kerry also said in a speech Friday that rockets carrying the chemical weapons “came only from regime-controlled areas and went only to opposition-controlled or contested neighborhoods." On NBC’s “Meet the Press” on Sunday, he said that hair and blood samples given to the United States by emergency workers showed signs of sarin.

The U.S. also accused the Syrian military of an attempted cover-up, shelling the area to remove evidence of environmental contamination.

U.S. and U.K. intelligence officials tell NBC News that Maher Assad, Bashar’s younger brother, authorized the attack, and Syrian rebels confirm that the 155th and 127th brigades of the 4th Armored Division, both of which are under his command, played key roles in carrying it out.

Status of investigation

The U.N. is currently investigating not just the attacks in Ghouta, but a March 19 attack on the rebel-held town of Khan Al-Asal, near Aleppo, which reportedly killed dozens. The use of sarin is suspected in both.

The Ghouta attack -- which the U.S. says killed 1,429, while the French estimate the death toll at "around 350" -- took place as U.N. inspectors were collecting samples from the Khan Al-Asal attack. The inspectors eventually gained access to the Ghouta site, and will ask to return to the scene of the earlier attack after the current investigation is complete.

According to the U.N., the blood and environmental samples collected in Ghouta were sent two European labs, which began testing on Wednesday. U.N. spokesman Farhan Haq said he could not provide a timetable for when the tests would be completed. But under rules of the Organization for the Prohibition of Chemical Weapons, which carried out the U.N. inspections, the labs have 15 days to test the samples for chemical weapons residue or telltale chemical compounds caused by breakdown of toxic agents, determine what type of chemical – if any – was used in the attack and report their findings.

If the tests are positive, the labs will not make a determination on who fired the chemical weapons. That would fall to the U.N. to investigate.

Richard Engel is NBC News' chief foreign correspondent; Jim Miklaszewski is NBC News' chief Pentagon correspondent; Robert Windrem is an investigative reporter.

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