IN A REMOTE corner of City Hall's seventh floor - a place of exposed pipes and unfinished halls far above the gilded cornices and ornate friezes of the ceremonial rooms below - there is an inconspicuous door with the words "WOOD-WORKER'S DIVISION" written on it in black marker.

Just inside, there is an office, some construction materials and graffitied walls. Further in, however, there is something less likely to be found on the seventh floor: a small room paneled with what appears to be bulletproof glass and signs that read, "DO NOT ENTER / INVESTIGATIVE GRAND JURY FILES."

About 90 file boxes, some marked "CRISIS" and "HEATING" with dates from 2005 to 2007, are in the room, which is bolted shut. A few inches off the floor in a corner is one of the small circular windows that line City Hall's mansard roofs.

So what are these grand-jury files and why are they in what is essentially the attic of City Hall, surrounded by offices of the Department of Public Property?

It was a mystery to Public Property Commissioner Bridget Collins-Greenwald, who said she wasn't aware of the sealed room but checked it out after being contacted by the People Paper.

The woodworkers shop, she said, is being used by Otis mechanics working on elevator-repair projects in City Hall. As for the files, no one in her office had an explanation.

"They're not ours. We didn't put them there," she said. "We do not have a key to that room."

The signs taped to the doors said to contact the Office of the Inspector General or Police Detective Charles Figorski in case of emergency.

District Attorney Seth Williams was inspector general for the dates listed on the boxes.

"I don't recall as the inspector general storing anything like that up there," Williams said.

Figorski, however, had the answers.

The files are from an investigation into the Low Income Home Energy Assistance Program (LIHEAP) that led to 18 people being charged for orchestrating fraudulent benefits to unqualified payees.

The files, which had to be under lock and key because they contain personal information on LIHEAP customers, originally were housed in an office rented by the Department of Licenses & Inspections. L&I gave up that office, and Figorski, who worked the case, found a new home for the files.

"No other place was big enough to store them," he said. "It was the only place that was secure."

Figorski said the city is allowed to destroy the files in two to three years.

So why was there a secured but unused room in the seventh floor in the first place?

Figorski said it was once a holding cell for prisoners standing trial in the building - before the Criminal Justice Center opened in 1995 and left City Hall with only civil cases. There is a dedicated elevator for the holding facility.

In those days, he said, many more cops worked at City Hall, and sometimes they would razz rookies who weren't used to the building.

"Officer so-and-so, report immediately to Room 721 for your meeting with the commissioner," a command would go out over the intercom.

Room 721 is a men's room.