The incident took place at Lynnway Auto Auction, a used car dealership, just over 20 miles (35 kilometers) north of Boston.
Police said there were "fatalities among the multiple victims."
US media said up to three people were killed and around a dozen injured when a jeep sped through a crowd, around 90 minutes after an auction was scheduled to begin.
"At this point, there is no evidence or information to suggest the incident was caused by an intentional or terrorist act," said Massachusetts State Police.
A police spokesman said an update would be provided at a news conference scheduled for 1:00 pm (1700 GMT).
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