LONDON (AP) — The chairman of the inquiry into a London tower bloc fire that killed 72 people wrote to Britain’s attorney general Thursday to ask for a pledge to prevent evidence from corporate witnesses being used in subsequent criminal prosecutions.

Martin Moore-Bick’s request to Attorney-General Geoffrey Cox is an important development in the investigation into the 2017 blaze at Grenfell Tower as corporate entities involved in a refurbishment before the fire had threatened to remain silent on the grounds they may incriminate themselves.

Such silence has the potential of hampering efforts to learn lessons from what was the greatest loss of life in a fire on British soil since World War II.

London’s Metropolitan Police are conducting a separate probe into crimes ranging from gross negligence to manslaughter to health and safety violations.

The second phase of an inquiry opened this week, but stalled quickly when the lawyer for the inquiry pointed out that submissions from the firms involved in the refurbishment expressed “no trace″ of responsibility for the catastrophic fire. Experts in the first phase of the inquiry concluded that the work failed to comply with building regulations.

The attorney for the inquiry, Richard Millett, advised Moore-Bick to ask for the pledge.

“Without it, you will not get to the truth,” Millett said.

But attorneys for the victims objected, arguing that the demand for immunity — together with the last-minute timing of the request — suggested the firms were trying to derail the effort to investigate the root causes of the disaster.

Lawyer Michael Mansfield said the “potential perpetrators of this inferno” are trying to “essentially dictate the terms in which they will provide their assistance.”

The firms argued such requests for immunity had occurred in other inquires, such as the one into the 1972 Blood Sunday shootings, at the height of the so-called Troubles in Northern Ireland.

High-rise apartment towers are supposed to be designed to stop apartment fires spreading, but within minutes of starting, the flames raced up the outside of the tower like a lit fuse.

The question is why and the inquiry continues.