(Photo by DERRY FIRE DEPARTMENT) Firefighters work from a pair of aerial platforms to seek out and extinguish remaining hot spots in the facade and interior rooms of the office building damaged by fire over the weekend.

DERRY — While the final report isn’t yet official, investigators are quite sure the 4-alarm fire that significantly damaged a Tsienneto Road office building late last week originated in “combustible” bark mulch — sparked by the improper disposal of smoking materials.

Three firefighters were injured during the fire, two of whom suffered hand injuries and one with smoke inhalation, but none of the injuries were considered serious and all three were treated and later released at a local hospital.

“Based on early-incident observations … combined with witness interviews and sourced photographs/video from bystanders and social media platforms, the area of origin was readily identified,” Fire Prevention Bureau director David Eastman and inspector Michael Garone said in a joint statement.

Once the area was identified, a scene-investigation was conducted and the cause was identified to be improper disposal of smoking materials, according to the statement.

That it was the “improper disposal of smoking materials,” which were “discarded into combustible mulch landscaping,” prompted the officials to remind the public “to never discard smoking materials in mulch or potted plants, and never toss cigarettes from porches or windows.”

(Photo by DERRY FIRE DEPARTMENT) The first Derry firefighters to arrive at the Overlook Medical Park at 6 Tsienneto Road found a column of fire climbing up the facade and working its way into the interior of the structure. Careless disposal of smoking materials in bark mulch sparked the blaze, officials said.

The cause has been classified as accidental, they added.

Primarily a medical office building, the three-story facility at 6 Tsienneto Road was occupied by staff and patients when the fire broke out, Eastman and Garone said.

“Employees and management of the Overlook Medical Park are credited with an orderly evacuation of the building,” they said, adding that their efforts “resulted in no injuries to the public.”

Dean Shalhoup may be reached at 594-1256 or dshalhoup@nashuatelegraph.com.