Clues to the source of the chemical poison that U.S. officials say killed more than 1,400 people in Syria last month may come from the weapon casing, not the material itself.

The source question boils down to who made the substance, thought to be sarin—the government or another actor. Secretary of State John Kerry today told Senators that he is “certain” none of the opposition has the weapons or capacity to make a strike of this scale, but distinguishing purified military grade sarin from sarin brewed up at home by studying traces left at an attack site, is not easy.

The two versions do differ in some ways. Military grade sarin, for instance, would not contain chemical byproducts likely to be present in sarin made through other recipes. And traces of the impurities in home-brewed sarin would be detectable in soil. But different recipes for the home-brewed version will yield different byproducts, and investigators may have no way of knowing which ones to seek. Even if the byproducts were known, detectives would need to know the normal levels of those compounds in the soil to determine whether the amounts are elevated.

Likewise, examining tissues from victims would provide little help. Sarin kills by interfering with the action of a nervous system chemical in a way that ends up overstimulating muscles and paralyzing those around the lungs, impeding breathing. Physicians can detect signs of sarin in the tissues and in urine.  Indeed, Secretary Kerry told the Senate Foreign Relations Committee today, "We have now learned that the hair and blood samples from first responders in east Damascus has tested positive for signatures of sarin." But investigators cannot discern the telltale impurities of the home-made nerve agent.

It is for such reasons that some experts say examining pieces of the rockets that the sarin arrived in is likely to be more telling, says Michael Kuhlman, chief scientist for national security at Battelle Memorial Institute, a nonprofit research group in Columbus, Ohio.  Fragments of the weapons will be on site in the same places where inspectors will be digging up soil samples for evidence of sarin, he says. The materials composing the rockets could differ depending on who made them, thereby pointing a finger at who deployed them.

Calculating how much sarin was used to create these mass casualties creates an additional challenge. Typically, a chemical attack like this would come in the form of a rocket a couple meters long topped with a nose containing a liquid form of sarin. The liquid would be transformed into a fine mist when an internal detonator blew up the rocket. How many people were hurt would depend on the geography it landed in­, the wind, the temperature and the population around it. The colder weather of the early morning hours during the August 21 attack, for example, would allow the vapor to remain longer than it would have persisted in the heat of midday.

In the videos released in the aftermath of the August 21 attack, first responders can be seen apparently tending to the victims without wearing protective gloves or masks, and so the total number of casualties could be larger than estimated from the direct attack alone. Sarin is known to cling to the skin and clothes and to be able to sicken individuals who encounter it that way  (as was the case among first responders referenced in a declassified French report released today). On the other hand, it’s not a given that helpers will get sick.  It may have been possible to help victims without becoming a casualty oneself, says David Moore, senior toxicologist in life science research at Battelle.  “It’s quite possible all those casualties were exposed to a very fine vapor of gas, and there was probably not large liquid contamination on those casualties once they were out of the environment and the gas had dissipated.” Those first responders may have experienced more minor impacts such as runny nose or eye pain that could remain for hours, he says.

Long-term impacts of high-level sarin exposure remain largely unknown. Research looking at lower level exposures from working at sarin production plants suggests, however, that sarin exposures can lead to effects including memory problems and sleep issues.