The star Olympian and Paralympian athlete was charged on Friday with the murder of 29-year-old Reeva Steenkamp who was shot dead at his luxury Pretoria home in a case that has gripped the world.
Police sources close to the investigation told City Press that Steenkamp's skull had been "crushed".
"There was lots of blood on the bat," one source told the paper.
Police are investigating whether the bat was used to assault Steenkamp, who was shot four times in the early hours of Thursday, or if she used it to defend herself.
Police have dismissed initial suggestions that Pistorius (26) could have mistaken Steenkamp for an intruder.
"The suspicion is that the first shot, in the bedroom, hit her in the hip. She then ran and hid herself in the toilet … He fired three more shots," a police source told City Press.
Pistorius — a national icon who inspired people around the world when he became the first double amputee to compete against able-bodied athletes in the Olympic Games last year — is spending the weekend in a police cell after being charged with murder.
He is due to apply for bail at a court hearing on Tuesday, the same day a memorial service will be held in Port Elizabeth for his slain girlfriend.
Pistorius, who had been going out with Steenkamp since late last year, faces a life sentence if convicted of premeditated murder, as alleged by state prosecutors.
Pistorius to plead not guilty
His family have indicated that Pistorius, who broke down sobbing during his initial court appearance on Friday, would plead not guilty.
"We have no doubt there is no substance to the allegation and that the state's own case, including its own forensic evidence, strongly refutes any possibility of a premeditated murder or indeed any murder at all," his uncle Arnold Pistorius said in a statement on Saturday.
Top experts have been brought in to aid Pistorius' legal team. The Sunday Times reported that a top pathologist, spin doctor and policing experts would be assisting his lawyer Kenny Oldwage and senior advocate, Barry Roux.
According to the newspaper, pathologist Dr Reggie Perumal was now on Pistorius' team.
Perumal was present when state pathologists conducted the post-mortem on Steenkamp.
Also on the team is spin doctor and former editor of the UK tabloid the Sun, Stuart Higgins.
Oldwadge told the Sunday Times the team was anxious to start working on the case.
"We are being prejudiced by not having been given access earlier. This is going to be an extremely long and busy week for all of us," he was quoted as saying. – Sapa