"Therefore, I'm sure that there will be surely the process of accountability when everything is over," Ban said.
The secretary-general thought his speech to the Women's International Forum and response to questions were not being broadcast, but they were shown on U.N. television.
Asked whether Ban's conclusion was in response to the report, U.N. associate spokesman Farhan Haq said that as far as he knows the report hadn't been completed "so it's not possible for any of us to have seen the report at this present moment."
But he added that Ban "has been in touch with different people including the experts."
Under the mandate for the inspectors, Sellstrom's team was to determine whether or not chemical agents were used and if so which agent not who was responsible.
But two diplomats said the report could point to the perpetrators, explaining that the inspectors collected many samples from the attack, which the Obama administration says killed 1,400 people, and also interviewed doctors and witnesses.
The inspectors had soil, blood and urine samples and may also have collected remnants of the rockets or other weapons used in the attack which could point to those responsible, the diplomats said, speaking on condition of anonymity because discussions on the issue have been confidential.
Speaking by telephone from the Netherlands, Sellstrom said he didn't know exactly when the report would be released publicly. He confirmed that "it's done, but when to present it is up to the secretary-general."
Haq said after the report goes to the secretary-general "he'll present it to member states, and we hope that we'll be able to provide it to you (the media) in short order."
In his speech, Ban said "the disaster" in Syria has created "a lost generation of children and young people" and led to "rising sectarian tensions, regional instability, the largest displacements of people in a generation, grave violations of human rights, including sexual violence."
"The latest fighting has also raised the specter of chemical warfare which, if confirmed by the U.N. investigation mission, would be an atrocious violation of international law," Ban said.
Associated Press Writer Karl Ritter contributed to this report from Stockholm.