A massive apartment building explosion in Silver Spring will be ruled accidental, officials said Monday, after investigators concluded that a maintenance worker cut through a gas line he mistakenly thought was a plumbing drain line.

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       That led to a build up of gas, causing a thermal “flash fire” followed by the blast, Montgomery County Fire Chief Scott Goldstein said.

       Firefighters searching the debris also are increasingly sure that no victims were buried there, Goldstein said.

       Work continued Monday at the Friendly Garden Apartments, a four-story section of which blew up Thursday morning, resulting in 14 people going to hospitals. Most had minor to moderate injuries. Three were more seriously injured, including a child.

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       “One of the things that we are extraordinarily blessed with from Thursday’s events is, as of now and remains the case, no loss of life,” Goldstein said.

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       The blast erupted at 10:30 a.m. when many residents were away at work or in school.

       Cut gas pipe found in basement after Silver Spring apartment explosion, officials say

       Goldstein said the worker was trying to fix a plumbing problem. The spark or flame that set off the gas appears is not known.

       “In any type of residential or commercial structure, there are many, many ignition sources,” Goldstein said. “Could be a pilot light. Could be a cigarette. Could be the simple flick of a light switch. Could be a motorized tool, or all kinds of components, could be that ignition source.”

       The blast also led to a large fire. Fire and police investigators eventually were able to account for all residents in the building as either being away from the building at the time of the blast, not seriously hurt or hospitalized. Still, there was concern that an unknown building visitor — a delivery person, perhaps — was dead under the debris

       Firefighters led specially trained dogs to react to the smell of deceased humans across the debris pile. One of the dogs “alerted” to such a scent Thursday, and a different dog alerted on Friday. But dogs working the debris pile later Friday and Saturday detected no such odors, Goldstein said.

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       The explosion destroyed a section of the apartment complex. It also damaged units in nearby buildings, such that about 120 residents will have to live elsewhere for the foreseeable future. Goldstein said that many of those residents, with firefighter escorts, were able to go back into their units over the weekend to retrieve critical items. He expects more residents to be escorted to their units Tuesday morning.