An Oxford mathematician was released on bail today after being grilled for 36 hour in the death of his best friend who was another Oxford professor.

Dr. Devinder Sivia was arrested in connection with the death of 50-year old astrophysicist Steven Rawlings, although police have not been able to determine how Rawlings died Wednesday night.

An autopsy was completed on Thursday but coroners were not able to pin down the cause of death.

"This is a tragic incident and our investigations are ongoing to establish the cause of death,"  Detective Superintendent Rob Mason said.

Mason emphasized that police are looking into "all potential circumstances that could have led to his (Rawling's) death.  We are mindful that ultimately the death may be a matter for a coroner's inquest rather than a criminal court."

Neighbors called police Wednesday night after hearing Rawlings and his mathematician friend fighting. When they arrived, Rawlings was unconscious. A neighbor tried to revive Rawlings using CPR, but he was pronounced dead at the home.

Neighbors said Rawlings and Sivia were the "best of friends."  The two had known each other for decades.  They wrote a book together in the late 1990's called "Foundations of Science Mathematics."

Police believe the professors had dinner at a pub Wednesday night, but it's not clear what sparked their late night argument.  Police arrested Sivia Wednesday night.  Police questioned him for 36 hours before releasing him on bail today.

"It must have been a terrible accident," Professor Rawlings' sister told British newspaper The Telegraph. "They have been friends for 30 years.  We can't think that there was any kind of fight."

Faculty and students at the prestigious Oxford University were in a state of disbelief. The Vice-Chancellor Professor Andrew Hamilton released a statement saying "The entire university community has been profoundly saddened and shocked by the tragic and untimely death of Professor Steve Rawlings.  Our thoughts are with his family and friends."

Rawlings was married.  His wife reportedly rushed back from an overseas trip after being told of his death. The two celebrated her 50 th birthday last week.  She has asked reporters to respect her privacy, and has not commented on her husband's death.

On his Facebook page, Rawlings lists his interests as books, films and world peace.  His last wall post was December 23 rd when he challenged friends to top the number of places visited around the world. "My competitive streak takes over again," he wrote.