Six individuals are dead and dozens are in critical care after a fast-burning blaze took over a tower in London, England.

Emergency responders were first called to the scene shortly before 1 a.m. Wednesday morning where they found that the speed and intensity of the blaze meant that they could not reach to the highest apartments in the building to rescue individuals trapped inside.

The Grenfell Tower was built in 1974 and is 24 stories tall. 120 homes are contained inside the tower, meaning that most may have been filled with individuals and families sleeping at the time of the fire.

Currently, six people have been confirmed dead by officials, with 20 others in critical condition. 74 people total have been admitted in nearby hospitals and are receiving treatment.

Many in the building are still missing and firefighters are still attempting to reach the top four floors to search for bodies or survivors. Firefighters who had been fighting the blaze for ten hours left this morning to be replaced by a relief team who continue to grapple with the smoldering building.

So far, officials have not given an official reason for the blaze, nor indicated whether it was accidentally or purposefully set. Firefighter experts say that the blaze was unlike anything they had ever seen.

Mark Coles, head of technical regulations at the Institution of Engineering and Technology, said that the rate at which the fire caught indicated that there were serious structural issues in the building.

“The speed at which this fire spread would suggest that there has been a serious failure in the design and installation techniques employed.”

According to Buzzfeed, Residents’ groups had previously lodged complaints and warnings about the safety of the building in regards to their fire safety preparedness.

A full investigation of the fire is expected.