"I know Philadelphians demand action," he said. "I've heard their voices loud and clear everywhere that I've gone this weekend." At the same time, Williams added, such an inquiry could take months.
Clarke said a new Council committee would hold hearings over the summer, write a report, and potentially draft corrective legislation.
Meanwhile, funeral and memorial services for the six victims of the collapse have been scheduled throughout the week, and Myra Plekan, the 61-year-old woman pulled from the rubble after more than 12 hours, continued to recover. Her condition was upgraded Monday from critical to serious, hospital officials said.
She was one of 13 people rescued from the store.
Griffin T. Campbell, the contractor responsible for demolishing three buildings in the 2100 block of Market Street, provided his first words since the collapse, offering his condolences in a letter released by his attorney, Kenneth Edelin.
Edelin asked "that there be no rush to judgment."
"Mr. Campbell is confident that the results of the investigation will reveal that professional and safety-conscious business practices were in place," Edelin said. "I am confident that when all the facts are known, Mr. Campbell will not be deemed responsible for the tragedy that happened."
Sean Benschop, whom Campbell hired to operate an excavator at the site, faces six counts of involuntary manslaughter and other charges. Benschop, 43, had marijuana in his system, according to a toxicology report.
Edelin said Benschop "has extensive experience in demolition," including "numerous demolition contracts with the city."
Mayor Nutter's spokesman, Mark McDonald, said Benschop had completed three city jobs and had two more pending.
While Campbell said on his application for a demolition permit that the Market Street work would cost $10,000, Edelin said Campbell's bid for the job was actually $112,000.
Nutter said Monday that city government can "always" do a better job of oversight, but most of the blame for the collapse should lie with Benschop, Campbell, and Richard Basciano, the owner of the buildings being demolished.
Basciano, 87, has long owned numerous properties in the 2100 and 2200 blocks of Market Street, including the now-shuttered Forum porn theater. He only recently decided to redevelop the area.
"It's that trio that I think most of the focus should be on, and where you're probably going to find most of your responsibility," Nutter said. "In the end, I think we may come to the conclusion that, but for the actions of the person operating the equipment, it's quite possible that wall would still be up."
On Friday, Nutter announced a series of changes to require better inspections of demolitions and quicker action to shut down dangerous sites.
He said he would ask Council to pass a bill requiring separate licensing, background checks, and drug screening for demolition workers.
Patrick Gillespie, business manager of the Philadelphia Building Trades Council, on Monday offered to assemble a "construction and demolition advisory committee," consisting of safety experts from Local 98 of the electricians union and the Laborers District Council.
In a letter to the mayor, Gillespie called his offer "a good faith, no-charge" effort to link union safety experts with city agencies charged with monitoring construction and demolition sites.
In addition to the grand jury investigation and the Council inquiry, several other probes of the collapse already are underway, including ones conducted by the police, the fire marshal, the Department of Licenses and Inspections, the city inspector general, and the federal Occupational Safety and Health Administration.
At least six of the victims have filed a lawsuit against Basciano, Campbell, and others. Their attorneys are gathering evidence from the site.
Williams, the district attorney, compared the pending grand jury investigation to one convened in 2011 to weigh charges against West Philadelphia abortion doctor Kermit Gosnell.
That grand jury report not only recommended criminal charges against 10 people, but analyzed the failure by state Health Department officials to regularly inspect Gosnell's abortion clinic.
The grand jury looking into the collapse will have similar freedom to examine the roles municipal agencies, private companies, and individuals might have played in the collapse, Williams said.
He warned that the investigation could take time - the Gosnell grand jury met for nearly a year - and urged residents to be patient.
Williams also announced a grand jury after a fire and collapse at a vacant Kensington mill killed two firefighters last year. Grand jury proceedings are secret, and the status of that probe is unclear.
Council's new committee - with members Curtis Jones Jr., Jannie L. Blackwell, Bobby Henon, James F. Kenney, and Maria Quiñones Sánchez - plans to take a broad view as well.
The committee may examine the practice of contractors using "third-party expediters" to pull permits; the budgetary constraints of L&I; the city's management of blighted and vacant property; and citywide standards for demolition contractors.
The committee could wander into a political tussle with Nutter. Clarke spoke Monday about the "pretty aggressive and quite comprehensive" standards for demolition work under a blight-removal program championed by Mayor John F. Street.
Nutter, a Street rival, spiked the program shortly after taking office in 2008.
Several Council members also have focused on "the underground economy" of day laborers paid in cash.
"All of us can probably tell you about walking down the street and seeing somebody standing on top of a property with a crowbar on the same roof they're attempting to take down," Clarke said. "Unfortunately, it took such a tragic event for us to finally do something about that."
Kenney, who said L&I's budget should be "as big as it needs to be to keep people safe," said demolition permits should be issued only to companies that are "qualified, recognized, trained, insured and drug-tested."
"You don't need as many inspectors to go out and check on them because they do the right work the right way," he said.
Contact Troy Graham at 215-854-2730 or firstname.lastname@example.org, or follow on Twitter @troyjgraham.
Inquirer staff writers Bob Warner and Summer Ballentine contributed to this article.