Two doctors working together said the cause of death was not lung infection but in fact "shock caused by being burnt alive".
Rajesh Pandey, a senior police superintendent, told local media of the doctors' verdict.
A police spokesman added: "This happens when someone is burnt alive. The particles go inside with the breath. If a person is dead, such particles cannot reach the lungs and the windpipe. So, the doctors concluded that the woman was burnt alive on the pyre."
In another development, the dead woman's maternal uncle Kailash Singh has reportedly accused her husband and ten others of sexually assaulting and killing her.
Police are taking the accusations seriously but all the men have disappeared, they say.
However, the doctors at Sharda maintain that Rachna did in fact die at their hospital and post-mortem doctor Pankaj Mishra says he cannot even be sure that the body he examined was that of Rachna, due to the extent of the burns on it.
Devesh told local media, from an unknown location, that his in-laws were trying to frame him and his relatives in a bid to acquire his property.
Doctors have also preserved a piece of bone for conducting DNA tests, which was handed over to the police. They did not say when DNA test results would be available.