District Attorney Seth Williams said the "scope and depth" of the grand jury process will help prosecutors, the city and others to "completely and appropriately investigate" the accident.
A building at 22nd and Market was being demolished when it collapsed onto a neighboring Salvation Army Thrift Store on Wednesday, killing two employees and four customers.
The grand jury will hear from witnesses, gather information and other documents as it looks into everything that preceded and allowed for the collapse, Williams said.
"I know Philadelphians demand action. I heard their voices loud and clear," Williams said at a news conference. "We want Philadelphians to be patient as we gather all the evidence."
Also Monday, City Council was planning to announce the formation of a special committee to conduct a broad review of procedures and regulations regarding licenses and permits, construction and demolition, the certification of workers, building maintenance and other issues.
In the fallout of the collapse, officials have begun inspecting hundreds of demolition sites citywide in recent days. Mayor Michael Nutter said Friday that the city was preparing to implement sweeping changes in its regulations of building demolition.
Police allege a heavy equipment operator was high on marijuana when the collapse happened. He surrendered Saturday to face charges in the deaths.
The excavator operator, 42-year-old Sean Benschop, faces six counts of involuntary manslaughter, 13 counts of recklessly endangering another person and one count of risking a catastrophe. His attorney said it was an accident and his client is not responsible for it.
A demolition permit indicates that contractor Griffin Campbell was being paid $10,000 for the job.
Campbell's lawyer, Kenneth Edelin, released the following statement Monday:
Mr. Campbell, and the Campbell Construction Company, grieves for the families who have lost loved ones. Mr. Campbell also prays for speedy recoveries for the fellow citizens of this city who were injured. The Campbell Construction Company joins with the Mayor, L&I, and OSHA in asking for a far-reaching and thorough investigation. As his attorney, I respectfully ask that there be no rush to judgment while this investigation is conducted. Mr. Campbell is confident that the results of the investigation will reveal that professional and safety-conscious business practices were in place at the 22nd and Market Streets site. I am confident that when all of the facts are known, Mr. Campbell will not be deemed responsible for the tragedy that happened. Again, our prayers and condolences are extended to the families and the injured.