"The forensic analysis did not find any explosives or inflammable residue in the burnt third-floor modular lab," sources quoting the forensic report said.
The forensic report was submitted to the police on Jan 8.
Citing lack of physical evidence, sources said that the chances of finding the exact cause of the incident is very remote.
They added that the burnt computers and hard disks are still being analyzed by forensic experts.
Earlier, Srikumar Banerjee, Atomic Energy Commission Chairman and BARC Director has said that possibility of vapour accumulation was being considered.
"We are also looking at the possibility of vapour accumulation. The problem is that everything was charred and not much trace of anything is left," he said.
Two PhD students, Umang Singh and Partha Bag were killed in the fire that broke out at the chemical lab at BARC on Dec 29, 2009.