New Delhi, Jan 7 (IANS): Contradictions in two autopsy reports about traces of alprazolam (an anti-depressant), in Sunanda Pushkar's viscera delayed the investigation and registration of the FIR, Delhi Police said Wednesday.

Police said there were two reports of autopsy, conducted on Sunanda's body by the board of three doctors at All India Institute of Medical Sciences (AIIMS) Jan 18, 2014.

The first report ruled out the possibility of alprazolam in her viscera while the final report which came in September suggested alprazolam poisoning.

"Both the reports mentioned traces of poisoning but the substance was not made clear. Prima facie, it was suspected to be a case of overdose of alprax tablets consumed by Pushkar as empty strips of alprax tablets were recovered from her hotel room, but first report ruled it out," an official told IANS.

The second report talked of alprazolam poisoning, he said.

On Tuesday, Delhi Police Commissioner B.S. Bassi revealed that they had registered a murder case, nearly one year after the mysterious death of former Congress minister Shashi Tharoor's wife Sunanda Pushkar in south Delhi's hotel room Jan 17, 2014.

The first report, a copy of which is with IANS, said: "Two used alprax strips of capacity 15 tablets each were recovered from the crime scene. However, viscera report is negative for presence of alprazolam."

The second report said: "The circumstantial evidence are suggestive of alprazolam poisoning."

Delhi police said that due to differences in two reports they made a request in the first week of Dec, last year to AIIMS's board, headed by forensics Sudhir Kumar Gupta, to give their final opinion.

Police Dec 28, 2014 received the final report.

Sudhir Gupta, told IANS: "We have also taken an independent opinion from the head of the department of a leading hospital of Delhi, department of pathology. So, this is a combined opinion of the AIIMS medical board."