"The regime is constantly changing its position and that reinforces doubts and suspicions and lends credence to demands by the PPP for an independent inquiry under UN auspices," Farhatullah Babar, PPP spokesman, said.
"If the PPP comes to power, we will ask the UN to hold an inquiry."
Musharraf has invited British detectives to help investigate the murder but has rejected the demands for a UN-led inquiry.
Video footage of the attack shows man armed with a pistol firing several rounds at Bhutto from close range as she waved to supporters from the roof of her car after a political rally in Rawalpindi.
None of the images clearly show her being hit, although from one angle her headscarf appears to flick out. Seconds after the assailant opened fire, a suicide bomber detonated a device near Bhutto's car.
Bhutto was buried the day after her killing, in keeping with Muslim custom, without a post-mortem.
"I feel that there is a skeleton in the cupboard of the government and that's why they have been changing their stance," Babar Awan, a senior PPP leader, said.
"From day one we have been saying that she was shot. It was a conspiracy, a plan executed, we have absolutely no doubts about it."
Javed Cheema, the interior ministry spokesman who announced the government's initial findings that Bhutto had not been shot, told the AFP news agency: "Whatever the president has said, it must have been right.
"I cannot add anything to it. The investigations are continuing now and when these are concluded we will share the findings with the public."
Musharraf also told CBS that Bhutto only had herself to blame for her death.
"For standing up outside the car, I think it was she to blame alone. Nobody else. Responsibility is hers," he said.
The assassination sparked violent protests by Bhutto supporters who blamed the government for failing to protect her despite requests for tighter security after a suicide bomber targeted her in October.
The unrest forced the postponement to February 18 of parliamentary elections, seen as a crucial step toward democracy after eight years of military-led rule.