By Joe Holden

PHILADELPHIA (CBS) — The burnt and collapsed portions of a West Chester senior living community are a familiar sight for some of the National Response Team members on the ground. Some of them have been to devastated sites like New York City and Oklahoma City.

Investigators say the five-alarm fire response Thursday night was off the charts.

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And now to sift through the rubble, a so called “recovery response,” largely assisted by the Bureau of Alcohol, Tobacco, Firearms and Explosives.

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Investigators are lifted in and out of the extensive scene by way of a yellow cage.

Thirty special agents are from the Philadelphia office, along with a national insertion team that responds to catastrophes and other high profile incidents.

“National Response Team (NRT) is a group of specially trained agents and technicians certified fire investigators, explosives, chemists, fire protection engineers, electrical engineers,” said Don Robinson, with the ATF.

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The federal detail is on loan to Chester County for the foreseeable future, CBS3 has learned.

“They are here from all over the nation and they are going to miss the holiday because they understand the magnitude of what went on here,” said Chester County District Attorney Tom Hogan.

The events of Thursday night take on a personal note for an agent of the ATF as they press forward to reach a conclusion of what happened here.

“I actually have an aunt who lives in this complex. She was able to be evacuated and be with family. We all have a piece of this,” Robinson said.

On Monday, Hogan said four people remain unaccounted for following the blaze.

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“This is a tragedy,” said Hogan. “We have four people unaccounted for and we’re not expecting good news.”