Remember that Giant Charlestown Fire? Here's What Caused It

Two buildings gutted. Six alarms called within a matter of hours. Ninety degree heat. Nearly eight hours to fully douse the flames. Three firefighters treated and transported to the hospital. Nine people rendered homeless. Five million dollars in estimated damage.

Twelve days later, the Boston Fire Department has completed its investigation into last month's huge Charlestown fire and determined the cause.

"Investigators determined most likely cause of Charlestown 6 alarm fire July 21st as careless disposal of smoking material," the Boston Fire Department tweeted Wednesday.

• You can read Patch's full coverage of the fire and see more images here.

Boston Fire said it came to that conclusion after eliminating other possibilities and conducting multiple interviews.

The fire, which poured smoke into the sky over Charlestown in late July, started at 284 Bunker Hill St. and spread to the units at 284 and 286.

Top photo courtesy Boston Fire Department.

Please note: This article has updated to correctly identify the address at which the fire originated. We regret the error.