The diplomats believe Sellstrom's team can figure out what happened from what one called "the wealth of evidence" they collected.
A determination of the delivery system used in the attack, and the composition of the chemical agent, could point to the perpetrator, they said.
The U.S. and its allies are certain the Syrian government is behind the attack though President Bashar Assad's government and its closest ally, Russia, have blamed the rebels.
U.N. associate spokesman Farhan Haq has said the inspectors would establish a "fact-based narrative" of the Aug. 21 incident.
The foreign ministers of France and Luxembourg have said that the report of the inspectors is expected on Monday.
But Haq could not confirm that onThursday, adding "the secretary-general has not received the report so far."
Haq said the U.N. has made some efforts to speed up the analysis, noting that instead of two laboratories, the samples are being tested at four laboratories in Europe. The testing could have taken three to four weeks, but the secretary-general has been pressing for a speedier report.
One diplomat said Russia was putting heavy pressure on Sellstrom to restrict his findings, but whether he does so remains to be seen. Secretary-General Ban Ki-moon could possibly go beyond the inspectors' findings and characterize who did it, the diplomat said.
If Sellstrom's report points to a perpetrator, there is certain to be demands for proof from the other side.
If the likely culprit is the Assad regime, diplomats say the Syrian government and Russia will almost certainly counter that the rebels have stolen or are manufacturing the equipment or material that the inspectors cite. Likewise, if the findings point to the rebels, their Western and Mideast supporters will almost certainly blame the government for fabricating the evidence.
Ban is expected to first report Sellstrom's findings to the U.N. Security Council, where the five veto-wielding permanent members have been discussing elements of a U.N. resolution that would demand that Syria's chemical weapons be put under international control and be dismantled, and condemn the Aug. 21 attack.
France's U.N. Ambassador Gerard Araud told reporters Thursday that the U.S., Britain and France have agreed on the basic elements of a draft resolution and discussed them with Russia and China at a meeting Wednesday but did not put a text on the table.