NEW YORK (CBSNewYork) – Con Edison says it has figured out what caused what caused a frightening explosion at an Equinox gym on the Upper West Side.

The utility company says an electrical service line — the the line that goes from the street to the customer — failed and impacted a gas line, leading to the explosion, reports CBS2’s Natalie Duddridge.

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Con Ed crews are digging in the area to find the exact place where the rupture took place.

Nine buildings will be without service and could be without gas until Monday night.

On Sunday afternoon, The first floor windows of Equinox health club at Broadway and 92nd Street have been boarded up after they were blown out in a frightening explosion.

People in the area said they could feel their apartments shake.

Firefighters had initially responded to the building for a report of a smoking transformer inside the gym where they discovered a gas leak. After the discovery, 90 people from Equinox and neighboring businesses and 15 people living above the Bank of America were evacuated.

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Shortly after local residents were cleared, a blast rocked the building.

“I saw like an orange flash and the. Followed up with an explosion,” said witness Rick Angeles.

“Gas is a very dangerous substance or any sort,” said FDNY Assistant Chief John Hodgens. Many things, something little as a telephone ringing or a light switch been turned on, can trigger an explosion. So that’s what triggered the explosion.”

Firefighters also made sure the animals inside the Petco next door were OK.

The FNDY shut off gas and electricity on the block while fire crews ventilated the buildings and checked carbon monoxide levels.

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The fire department is also working with the Department of Buildings to determine if the building is structurally safe.