MONROE — Fire officials say a 154-year-old home on Route 135 here is not safe enough for inspectors to enter to try to determine the cause of Thursday’s fire that destroyed the house owned by Scott Leonard — a former local firefighter — and his wife Lorraine, who now live in New York.

“The structure is extremely unstable,” Monroe Deputy Chief Russ Brown said Saturday. He said the devastation of the interior made it unlikely an exact cause would be established.

Brown said he accompanied New Hampshire Fire Marshal’s Office investigator Stacy Dubois Friday as they examined as much of the Civil War-era cape as they could. Brown said the home was “still standing,” at least for now.

“We looked in from the outside. We’re just not going to let anyone in there,” he said.

It was intense heat and thick smoke, as well as an increasingly unstable structure, that kept firefighters out Thursday on orders of Woodsville Fire Chief Brad Kennedy, who was in charge at the scene.

The midday fire was well under way when firefighters arrived. Officials said a metal roof hampered several of their attempts to create a vent through which heat and smoke could escape the structure.

Brown said fighting a mid-week, daytime fire is always more of a challenge because there is generally a lack of firefighters.

The Leonards’ son, Nathan Leonard, 20, along with Logan Onks, lives in the house, but were not at home when the fire started.

About 20 firefighters from Monroe, Woodsville, North Haverhill, Bath and Littleton, and Vermont departments from Wells River, Barnet and Peacham, battled the blaze.

“We’re really grateful that they were all able to get here,” Brown said.

bobhook@juno.com