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Investigators in two Massachusetts cities were seeking the renters of a house for sale that was intricately wired to explode and cause "significant destruction" if someone had simply flipped a light switch, police said Tuesday night.

The homeowner, his attorney and an electrical inspector were conducting a pre-sale inspection when the electrician discovered the mechanism, Milton Police Chief Richard G. Wells Jr. told reporters Tuesday. Boston police bomb specialists worked in the house, on Craig Street in Milton, for several hours before declaring it disarmed and safe.

"It took some work to put it in there," said Wells, who described the mechanism as involving wires meticulously snaking through several rooms from a gallon container of an incendiary substance, which was "secreted inside the house," to a particular light switch.

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"We believe the intention was that if someone had flipped the light switch on where it ended, the device would have exploded," he said.

Follow This Story on New England Cable News: House for Sale Was Wired to Explode

Milton police were called on a vandalism complaint last week to the house, which was being occupied by renters. When they arrived last week, officers found that the house's drains had been plugged with cement.

Both the Boston and Milton police departments are seeking the renters, who appeared to have abandoned the house, as persons of interest in the investigation, authorities said Tuesday night.

"I have never seen anything like this," said Wells, who said the would-be bomber or bombers "definitely had malicious intent in what they did."

Bomb squad technicians "took this very seriously," he said. "Clearly, by what we believe was the intent of the design of this device, it was definitely intended to do some significant destruction."

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