London (CNN) London's Metropolitan Police Service is considering manslaughter charges among the criminal offenses that may have been committed in relation to last week's deadly Grenfell Tower fire.

The police investigation is focused on how the blaze started, how it spread so fast and whether any person or organizations should be held responsible, Detective Chief Superintendent Fiona McCormack told a press briefing Friday. Documents have already been seized, she said.

McCormack said the number of people dead or presumed dead remains at 79 , but she fears the true number could be higher. She appealed for people to come forward with information about anyone who may have been in the building on the night of the fire.

UK authorities have given assurances that they will not check anyone's immigration status as a result of information given to police in relation to the blaze. The 24-story high-rise was home to 125 families, but visitors may also have been in the building when the flames took hold.

Witness Michael Kyriakou told CNN the fire spread quickly, with one side of the building ablaze around 15 minutes after it started. "Within an hour it had engulfed the top part of the building," he said. "There are people in bathrobes and slippers all around us, so hopefully as many as possible got out."

Witness Michael Kyriakou told CNN the fire spread quickly, with one side of the building ablaze around 15 minutes after it started. "Within an hour it had engulfed the top part of the building," he said. "There are people in bathrobes and slippers all around us, so hopefully as many as possible got out."

Residents from nearby Barandon Walk wait outside their building. The building was evacuated around 2 a.m. Police told residents that heat emanating from the fire could affect the structure of their building.

Residents from nearby Barandon Walk wait outside their building. The building was evacuated around 2 a.m. Police told residents that heat emanating from the fire could affect the structure of their building.

Smoke could be seen billowing over the heads of residents who gathered in nearby streets to watch the blaze.

Smoke could be seen billowing over the heads of residents who gathered in nearby streets to watch the blaze.

The building, built in the 1970s, was home to 125 families.

The building, built in the 1970s, was home to 125 families.

Children wear masks that were distributed near the site of the fire.

Children wear masks that were distributed near the site of the fire.

Police officers asked people to step back so they could expand the cordon and make more space for emergency services.

Police officers asked people to step back so they could expand the cordon and make more space for emergency services.

Witnesses reported terrifying accounts of people trapped in the tower. Some people were reported to have jumped from the tower.

Witnesses reported terrifying accounts of people trapped in the tower. Some people were reported to have jumped from the tower.

Burning debris falls from Grenfell Tower as a massive fire engulfs the London apartment building early on June 14, 2017. Seventy-two people are confirmed to have died in the fire.

Burning debris falls from Grenfell Tower as a massive fire engulfs the London apartment building early on June 14, 2017. Seventy-two people are confirmed to have died in the fire.

The investigation is one of the most complex ever undertaken by the Metropolitan Police, McCormack said. Work at the scene of the fire is "difficult and distressing," but search teams are endeavoring to recover everything possible so it can be returned to victims' families and survivors.

"Such is the devastation inside, our forensic search and recovery may not be complete by the end of the year," she said. "There is a terrible reality that we may not find or identify all those who died due to the intense heat of the fire, but we will do absolutely everything we can with the utmost sensitivity and dignity."

Failed safety tests

Speculation has focused on the role that cladding used in a recent refurbishment of the tower may have played in the fire, which appeared to spread quickly up the building's exterior in the early hours of June 14.

Samples of insulation from the tower and equivalent aluminum composite tiles sent by police for analysis have failed safety tests, McCormack said.

"Such are the safety concerns with the outcome of these tests we have immediately shared the data with the Department for Communities and Local Government, who are already sharing that information with local councils throughout the country," she said.

Investigators are also looking at the insulation behind the cladding and how the tiles were installed, she said.

The ruined Grenfell Tower, seen Friday in London, was destroyed by fire last week.

With the inquiry now in its second week, police have started taking witness statements from those who were in the tower, McCormack said, and have listened to all of the more than 600 emergency calls made that night to get a full understanding of how the fire spread.

"Some of those calls are over an hour long and truly harrowing in their content," she added.

McCormack also urged any residents of Grenfell Tower who had previously reported or had concerns about the building's safety to get in touch with police.

"We will identify and investigate any criminal offense and, of course, given the deaths of so many people we are considering manslaughter, as well as criminal offenses and breaches of legislation and regulations," she said.

The fire was not set deliberately but started in a Hotpoint fridge-freezer that was not previously part of any product recall, McCormack added. Police are talking to the manufacturers.

In a statement, Whirlpool -- which owns Hotpoint -- offered its condolences to the fire victims and said it was "working with the authorities to obtain access to the appliance so that we can assist with the ongoing investigations."

Suspect cladding found on 11 high-rises

The UK government said Thursday it was carrying out tests on 600 high-rise buildings across England that are covered in cladding, with at least 11 high-rise buildings so far identified as having combustible panels.

The affected buildings, housing potentially thousands of residents, are in eight local authority areas, including Manchester, Plymouth and Camden in north London, Communities and Local Government Secretary Sajid Javid said in a letter to lawmakers.

Addressing Parliament earlier Thursday , Prime Minister Theresa May said a statement on the test results of the cladding samples taken from Grenfell Tower would be made in the next 48 hours.

She added that the government could test more than 100 samples a day, through the Building Research Establishment, with the results coming within hours.

In total, 151 homes were destroyed in the fire, May said. So far, 164 properties have been found to house displaced residents and are currently undergoing inspection, she added.

Members of the emergency services work on the top floor of the gutted Grenfell Tower on June 17.

Camden residents fear fire threat

Camden Council said it was preparing for the immediate removal of cladding from five tower blocks on its Chalcots Estate after tests revealed that the panels were made of aluminium with a polyethylene core -- similar to those used on Grenfell Tower, although fitted differently -- and that it had introduced extra fire safety measures in the interim.

These include round-the-clock fire patrols on estate corridors and additional fire safety checks, it said. The council said it had told the contractor that installed the panels it was taking urgent legal advice.

The hole where a panel of cladding was removed from a tower block on the Chalcots Estate in north London.

"I cried when I heard the news, I was in shock," Chalcots resident Simon Morris told CNN. "I still am shocked, but along with the other residents I'm suffering a combination of shock and anger."

Fellow resident Sayed Meah said: "We've never had any fire evacuation rules, any plans, any procedures, nothing. So we're scared, we're genuinely scared (for) our lives."

Grants for survivors, victims' families

Additional financial help was announced Friday for victims' families and survivors of the fire in the form of charitable grants

The next-of-kin of those killed will receive an initial £20,000 (about $25,400), while those seriously injured will be given up to an initial £10,000, according to a statement from the Charity Commission. Those made homeless will get a "fresh start" grant of £10,000 once they are permanently rehoused, it said.

The money will come from several bodies which have been coordinating fundraising efforts for the victims. More than £10 million has been raised so far.

The government has also promised medical and financial aid to those affected, as well as committing to rehousing them within three weeks.