An artillery shell recently found in Iraq that contained three to four liters of the deadly nerve agent sarin (search) is "old and unmarked," senior defense officials told Fox News on Wednesday.

Officials said efforts to determine the exact age and origin of the sarin shell are picking up steam and that samples collected following Saturday's explosion of the shell are being sent out to several laboratories in the United States and the United Kingdom.

"... It is likely the IED (search) [improvised explosive device]-maker did not know he had sarin on his hands," said one line in the latest incident report made available to senior military officials at the Pentagon.

"One of the most pressing questions remains — Where did these IED makers find the shell?" another said.

Sarin is a colorless and odorless gas and is lethal in doses as small as .5 milligrams. Experts say sarin is more than 500 times as toxic as cyanide (search). Its current use is predominantly as a military chemical nerve agent.

This chemical weapon is most known for the March 1995 terrorist attack by the Aum Shinrikyo (search) religious cult, when members released the gas at several points in the Tokyo subway system (search), killing 11 and injuring more than 5,500.

The artillery shell recently found in Iraq was being used as an improvised roadside bomb, the U.S. military said Monday. The 155-mm shell exploded before it could be rendered inoperable, and two U.S. soldiers were treated for minor exposure to the nerve agent.

Three liters is about three-quarters of a gallon; four liters is a little more than a gallon.

"A little drop on your skin will kill you" in the binary form, said Ret. Air Force Col. Randall Larsen, founder of Homeland Security Associates (search). "So for those in immediate proximity, three liters is a lot," but he added that from a military standpoint, a barrage of shells with that much sarin in them would more likely be used as a weapon than one single shell.

The munition found was a binary chemical shell (search), meaning it featured two chambers, each containing separate chemical compounds. Upon impact with the ground after the shell is fired, the barrier between the chambers is broken, the chemicals mix and sarin is created and dispersed.

Another shell filled with mustard gas (search), possibly also part of an IED, was discovered on May 2, Defense Department officials said.

Some experts had suggested that the two shells, which were unmarked, date back to the first Persian Gulf War (search). The mustard gas shell may have been one of 550 projectiles that Saddam failed to account for in his weapons declaration shortly before Operation Iraqi Freedom (search) began. Iraq also failed to account for 450 aerial bombs containing mustard gas.

It's not clear if enemy fighters simply found an old stockpile of weapons, or if they even knew what was inside.

Military and intelligence officials said tests on the sarin shell will take several more days at least.

Officials have repeated to Fox News that throughout the late 1980s and early in the 1990s, Iraq manufactured 155-mm artillery and mortar shells containing chemicals that, when mixed, create sarin, as well as similar shells containing mustard gas and ricin, all with standard capacities of two to five liters.

U.N. weapons inspectors operating in Iraq until the mid-1990s reported that they could not account for some 550 such shells.

Fox News' Bret Baier and Ian McCaleb contributed to this report.