Devinder Sivia was detained after Steven Rawlings, a fellow of St. Peter's College at Oxford University, was found dead at his home Wednesday night. Investigators said an autopsy had not been able to determine how Rawlings died, The Daily Telegraph reported.
Rawlings' wife, Linda, and his older sister, Linda Davey, both said Friday they did not believe he was a homicide victim. Davey said her brother, 50, and Sivia, 49, had been friends for 30 years.
"I do not believe that Steve's death is murder and I do not believe Devinder should be tarnished in this way," Mrs. Rawlings said in a statement released by Thames Valley Police.
Sivia is a lecturer in mathematics at St. John's College. He and Rawlings wrote a book together, "Foundations of Science Mathematics," published in 1999.
The two men had apparently returned to Sivia's house in a village west of Oxford after dinner at a pub when Rawlings died.
Investigators originally believed the two had a violent quarrel.
Rawlings was an astrophysicist.