The US, UK, Israel and others have been collecting evidence to try to determine whether chemical weapons have been used.
The signs so far are that they have been, but politicians are being cautious of over-selling their level of certainty.
This is partly because of the lesson of Iraq when too much was based on too little hard information and all the caveats and cautions surrounding intelligence were lost.
And also partly because this time the political context is different.
With Iraq a decision had been made to go to war and the intelligence was brought into the public domain to make the case for it. This time political leaders - especially in Washington - seem much more reluctant to intervene and so the emphasis is precisely on the caveats and cautions.
Officials are saying there is some evidence of the use of chemical weapons but the US has talked about the need for further confirmation and a UN investigation before being sure.
So what would that look like?
Normally, the Organisation for the Prohibition of Chemical Weapons (OPCW) carries out inspections when there is a suspicion of chemical weapons being used.
In that case a carefully defined process is followed.
- None of the inspectors can come from a country complaining or being investigated
- They will document the process of gathering samples using video and photos to confirm what was collected in each place
- These samples are marked so that a chain of custody can be established to ensure nothing can be interfered with before they reach a laboratory certified by the OPCW
- A sample is sent to at least two labs
- If their assessments coincide then it is considered sufficient proof to go to the OPCW executive council where a state being criticised could challenge the evidence.
- If the executive council finds the case still stands then it could be referred to the UN Security Council
In the Syrian case, however, a special team has been organised under a UN mandate and leadership rather than the OPCW.
The team is 15-strong and headed by a Swedish scientist, Ake Sellstrom.
Two of the team members are pre-positioned in Cyprus and the whole team is ready for deployment within 24-48 hours of consent being given by the Syrian authorities.
However that is where the problem lies.
Search for samples
The UN wants consent to carry out an unconditional, unfettered investigation. It wants to be able to investigate all credible allegations - not just the ones the Syrian authorities allow it to see.
In the absence of a green light to go into the country, the team has been analysing information and trying to collect what it can from outside the country.
Two locations specifically identified are Aleppo and Homs, which are clearly dangerous.
Unlike a search for traces of radioactivity that can be done very quickly, looking for traces of chemical and biological agents can take longer, adding to the risk.
The aim of the mission, UN officials stress, is to find out whether chemical weapons have been used and not by whom.
There is a range of ways of looking for evidence of chemical weapons use.
Soil samples can be taken in which you can look for either residue of a chemical agent or the product's characteristic of when an agent degrades.
The other main route is through testing bio-medical samples.
The value of these depends on the time difference between the alleged attack and a sample being taken.
In the first few days, urine samples are useful to look for exposure to chemical agents.
After that time frame, urine becomes less reliable although blood samples can still be used.
However, this requires more sophisticated technology to look for agents in DNA, which can be stable and present for weeks after exposure.
Only a few labs in the world are capable of this, including in the US and at Porton Down in the UK.
Hair can also be examined, partly because it will pick up tiny traces of a chemical agent in the atmosphere.
No-one wants to get into the Iraq situation where major decisions are based on limited intelligence. But given the problems of getting access to a war zone to gain conclusive evidence, finding absolutely definitive evidence may be hard and may take time.
This may buy politicians in Washington and London time to work out what to do if something is found.