THOMASTON, Conn. – A swath of slate roof, 150 feet across, gave way and crashed. Bricks shot out for 20 yards. Timbers splintered and steel twisted. On June 15, a section of the sprawling former Plume & Atwood brass mill collapsed, casting doubt about the industrial property’s future.

Volunteers enjoying a hot dog and hamburger cookout at the nearby Railroad Museum of New England heard a rumble, then saw a cloud of smoke.

In a minute, another of the Naugatuck Valley’s brick industrial buildings fell closer to a likely demise. The mills and factories that employed thousands through the late 19th and 20th centuries have burned, fallen apart or been torn down.

Examples abound. The former American Brass complex in Waterbury’s South End is slated for demolition. Nova Dye, a former knitting mill in Waterbury, went up in spectacular flames, leaving a charred hole in the neighborhood. Along Franklin Street in Torrington, the former Torrington Manufacturing Co. was razed in 2010. Weeds grow through a stubble of broken bricks, behind a chain-link fence, a scar along the Naugatuck River.

Others sit in graffiti-sprayed hulks, including the former Bristol Company building in Waterbury. A former employee touring the dilapidated site a few years ago warned he might cry at the sight. It stands there, 310,000 square feet across 6.5 acres declared an environmental mess.

What happens to Plume & Atwood remains a question. The collapsed section can be seen from the East Main Street bridge or from within the 10-acre compound. The collapsed section of roof rests in a pile of snapped wood and crushed bricks inside. A window-lined exterior wall, just 20 feet from the railroad tracks, appears undamaged.

Still, the fire marshal and building inspector have halted northbound trains because vibrations might rattle the unstable building and cause further collapse. The Naugatuck Railroad runs sightseeing trains north to the Thomaston Dam, where the trains reverse and return to the station after a trip to the Waterville section of Waterbury. The closed tracks cut the ride about 10 minutes short, said Celeste Echlin, president of the Railroad Museum of New England.

Vance Taylor, a commercial real estate agent who has been involved with the property for more than 10 years, said efforts are underway to remove the portion of the building that collapsed. He said the owners are working through state and local approvals and permitting.

“The owner and I will reassess once the structurally unsound part is cleared away,” he said.

Plume & Atwood owner Jay Horowitz, who did not immediately respond to requests for comment, has spent more than $5 million on purchasing the complex and cleaning up its contaminants over the past decade. Between 2005 and 2006 asbestos was removed from pipes, oil and tar from walls, trenches and pits, and soils and groundwater were tested for contaminants.

Taylor said he has sought assistance for redevelopment from the Connecticut Trust for Historic Preservation. Taylor said the trust has a structural engineer who will look at the property and ensure there are no further collapses.

“It would be the owners’ desire to ensure its preservation and adaptive, creative reuse,” Taylor said.

Renovation of northwest Connecticut’s industrial sites has generally left few original buildings standing. The former Scoville Brass Mill in Waterbury is now the Brass Mill Center mall; Seymour Brass Co. in Seymour is a Stop & Shop, and the site of Ferrell Brass Mill in Derby is now a Home Depot.

Torrington’s Warrenton Mills, a former cloth mill turned into apartments in 1987, is a rare example of success.

“They make for very impressive buildings … but those are very few and far between,” said Samuel Gold, a senior planner for the Central Naugatuck Valley Council of Governments.

The Plume & Atwood property’s decline dates to 1993, when then-owner Diversified Industries filed bankruptcy and closed the plant, leaving behind $148,000 in unpaid property taxes.

Michael and Maria Blakeslee, then owners of M & M Trucking and Sanitation, bought the land and its 23 buildings for $355,000 in 1996. They had planned to grow their trash hauling business and develop part of the property into shops, small museums and a roller skating rink. Three years later, police investigated their company, which hauled residential trash. They were arrested in 1999, and Michael Blakeslee later pleaded no contest to fraudulently billing the town for trash from private customers not included in the town contract. He later sold Plume & Atwood and its 23 buildings to Horowitz, of East Quoque, N.Y., for $1,225,000.

Horowitz, owner of Lewis Brass Co., which owns American Unibrass Tube Co., a company still operating in a 65,000-square-foot building at the back of the property, brought that business to town in 1999 and bought the property a year later under the name P & A Realty.

In 2008, just before the stock market collapsed, Litchfield developers proposed a $50 million project to turn some of its 220,000-square-feet of brick buildings into a retail shopping outlet, condominium, and warehouses, but that never got off the ground.

“It really is unfortunate,” said COG planner Gold. “Those buildings at Plume & Atwood are really beautiful.”

Finding enough capital to take on projects the size of Plume & Atwood is a major deterrent to future development, Gold said.

“Rehabbing these buildings can be more expensive than new construction,” he said.

Prefabricated metal buildings can be built on concrete slabs in a matter of weeks, while pointing mortar around thousands of bricks, repairing roofs, and rewiring and plumbing buildings dating back to the 1860s is cost prohibitive.

But “as these buildings decay, we are losing a little bit of our industrial heritage” Gold said. “They don’t build buildings like those any more.”

The deteriorating mess is far removed from Plume & Atwood’s storied history, which started with the town’s namesake, Seth Thomas, who needed a local brass supply when he switched the inner workings of his clocks from wood to metal in 1852. First named Thomas Manufacturing Co., the factory turned out brass clock movements until 1870, when Waterbury brass barons David Plum and L.S. Atwood expanded their business north.