Someone’s careless disposal of a cigarette was the cause of a six-alarm blaze in Dorchester that caused an estimated $1.2 million in damage and left 30 people homeless, the Boston fire department said today.

The fire Wednesday night began on the third-floor rear porch of 10 Fox St. It spread to 12 Fox St. and 8 Juliette St., the department said.

The blaze struck around 6:35 p.m. Wednesday. Fire officials said the fire was a “perfect storm” because the three-deckers were close to each other and the fire swirled in a confined space.

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