WILMINGTON, Del. (CBS) —  Two firefighters were killed and four others were injured, two critically, while battling a row home fire overnight in Wilmington, Delaware.

The fire broke out shortly before 3 a.m. Saturday at a home in the 1900 Block of Lakeview Road.

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Firefighters encountered heavy flames as they entered the front of the residence and made their way upstairs. Wilmington Fire Chief Anthony Goode said the floors became weak and brittle, and began to collapse.

“There was a significant amount of heat and flames in the building, as well as illuminating from the windows and through the roof,” the fire chief said. “The floor fell in, four firefighters fell down and were injured in the collapse of the floor.”

Goode said two of those firefighters were killed. He identified the fallen as 41-year-old Lieutenant Christopher Leach, a 14-year veteran of the Wilmington Fire Department, and 51-year-old Jerry Fickes, who was on the job for 13 years.

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“Please keep us in your prayers,” he said. “This is a very tragic day for the Wilmington Fire Department to lose two individuals, two of our brothers.”

Goode said both fallen firefighters leave behind children.

He said four other firefighters were injured. Two of them were hospitalized in critical but stable condition. Two others were treated and released.

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The cause of the fire was under investigation.