The Seattle Fire Department is investigating how a massive fire started in Ballard Monday afternoon. Officials say the cause remains undetermined, but they think it’s likely connected to overheated electrical equipment. The three-alarm fire burned four businesses near the corner of Northwest Market Street and 24th Avenue Northwest. A fifth was damaged by smoke and water. “When I came by yesterday it was only smoke coming out of the jewelry shop. Little did we know what was going to happen,” said Daniel Shurman, one of the onlookers as Seattle Fire worked on scene Tuesday. Two-thirds of on-duty Seattle firefighters — about 150 — were on site battling the blaze Monday. One was injured and sent to Harborview for treatment.