A Philadelphia grand jury will investigate the Kensington blaze that killed two firefighters last week to determine if criminal charges are warranted, the District Attorney's Office said Tuesday.

Mayor Nutter, who has sharply criticized the owners of the former Thomas W. Buck Hosiery building for not responding to safety citations issued by the city, said he supported the grand jury's involvement.

"It will help us to get some answers to a number of unanswered questions. . . . It is absolutely the most appropriate next step," Nutter said. "We need to get to the bottom of what happened out there."

Michael A. Schwartz, attorney for the Lichtenstein family, which owns the property, declined to comment Tuesday, but he said his clients were cooperating with the District Attorney's Office.

Nutter said that all city agencies would cooperate with the investigation and that the city was conducting its own probe. The fire, he said, is a chance to "send a message" to other property owners that they must take responsibility for making their buildings safe.

An investigation into the cause of the fire is pending. Fire Commissioner Lloyd Ayers said all findings in the case would go to the grand jury.

Bill Gault, president of Local 22 of the International Association of Fire Fighters, applauded the decision and lashed out at "the absentee landlord who allowed the warehouse to become a death trap."

"We lost two of our own, and we'll never get them back," Gault said in an e-mailed statement. "We can, however, send a zero-tolerance message to these reckless landlords that we're coming after you and holding you criminally responsible for your willful negligence in the hopes that a tragedy of this magnitude never happens again."

The five-alarm fire at the former mill erupted early April 9 and spread quickly as high winds whipped the flames into an inferno. Philadelphia Fire Lt. Robert Neary and firefighter Daniel Sweeney were killed when a wall collapsed in an adjoining furniture store, where they were containing the fire.

Neighbors said they had complained to the city about the building's condition numerous times and feared it would go up in flames.

The owners of the building, Michael, Nahman, and Yechiel Lichtenstein of New York City, had been cited three times since November by the Department of Licenses and Inspections for failure to secure the property against vandals and squatters. The old hosiery factory was structurally sound; the owners planned to redevelop the property into 81 apartments and had an active zoning permit.

Lynne M. Abraham, a former Philadelphia district attorney, said there were too many unknowns to judge the likelihood of charges in the case - including the question of what caused the fire. Grand juries, she noted, can go in any number of directions to investigate the fire; they can subpoena witnesses and documents related to the case.

She said the closed-door, deliberate process was "a great way to proceed."

"I think the law works best in a calm, dispassionate fashion," she said.

After the collapse of Pier 34 in 2000, when three women drowned in the Delaware River, the pier owner and operator of a nightclub on the pier both were charged with a host of crimes, including involuntary manslaughter.

In that case, prosecutors argued that the men had been warned the pier was in imminent danger of collapse; the defense disagreed that they had received such a warning. One eventually pleaded to misdemeanor charges. The other entered a no-contest plea.

Frank DeSimone, a former prosecutor who represented the Pier 34 club operator, said Tuesday that he thought prosecutors would need more than three ignored L&I violations to seek similar criminal charges against the Lichtenstein family. He said the case was more like one in which a car accident was caused by someone who failed to pay attention rather than someone who was drunk.

"Is it so wanton and so beyond the pale that you would charge criminally?" he asked. "There is no intent here, so it would have to be gross negligence to overcome the lack of intent."

L. George Parry, another prosecutor-turned-defense lawyer, said a misdemeanor charge of failure to prevent a catastrophe might apply, since the building's owners were put on notice by the L&I citations and apparently took no action. That crime carries the potential for two years in jail.

Parry also predicted it would be difficult to charge the Lichtensteins with some of the crimes the Nutter administration has suggested, such as risking a catastrophe and criminal negligence.

"I think that would be a tough sell," Parry said. "But you never know what a creative prosecutor can do."

Contact Allison Steele at 215-854-2641 or asteele@phillynews.com.

Inquirer staff writers Laura Cofsky and Miriam Hill contributed to this article.