Nine other people were hospitalised -- including two with life-threatening injuries -- after the warehouse crash at Lynnway Auto Auction, a used car dealership in Billerica, just over 20 miles (35 kilometers) north of Boston.
"The preliminary information we have right now suggests that this is a tragic accident," Middlesex District Attorney Marian Ryan told reporters.
"At about 10:13 this morning police received a call of a vehicle that had gone out of control," she said.
A Jeep Grand Cherokee "suddenly accelerated and traveled through the building at high speed," said Ryan. It struck members of the crowd before crashing into a cinder block wall at the far end of the warehouse.
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"One male and two females have died as a result of their injuries," said Ryan. Nine other people were being treated at a hospital, including two with life-threatening injuries, she added.
The driver is a man in his 70s who works for the company. Reports suggested that he may have suffered a medical emergency, but Ryan said he was not hospitalized.
"All evidence and information at this time suggests an accidental cause.