Pakistan was plunged into turmoil last Thursday following the killing of Bhutto, who had twice been the country's prime minister and was about to run in parliamentary elections January 8 as leader of the Pakistan People's Party (PPP).
Cheema said investigators had unearthed major evidence, including witnesses' reports, and pledged to keep investigation transparent.
"All results of the investigation will be made public," he said.
Cheema said the authorities were doing their best to ensure the security of politicians, including opposition activists. Cities and areas where the most violent protests were reported have been placed under tightened police control.
The southern province Sindh, where the Bhuttos come from and where Benazir was most popular, was worst damaged by clashes.
According to official police statistics, at least 58 people have been killed and another 90 wounded in violence across the country since Bhutto's murder. Crowds of protesters burned 185 bank offices and damaged another 32 bank outlets. In addition, 240 government offices, 38 filling stations, 72 railway cars, 33 railway stations and platforms, 830 shops and over 570 vehicles were burnt or damaged.
Election authorities are to decide on Wednesday whether to postpone the parliamentary election, but a two-month delay is widely expected in the country.
