National Weather Service meteorologists have yet to complete their investigation into the damage, however, and may still upgrade their determination, The Boston Globe reported Friday.
"It is possible it could end up being ranked higher. But at this time, we feel comfortable releasing the information," meteorologist Robert Thompson said.
"It was at least an EF3 in the hardest-hit areas along its track, which certainly includes Monson."
EF3 is considered a strong tornado, with winds of 136-165 mph that can cause significant damage to many structures and destroy some that are not well-constructed.
Urban search-and-rescue teams have stopped searching affected Massachusetts communities for survivors trapped under wreckage because there are no reports of missing persons, Peter Judge, spokesman for the Massachusetts Emergency Management Agency, said.
"There are, officially, no missing persons. There is no fear that a number of folks are under the rubble," he said.
State and local crews were clearing away downed power lines, fallen trees and damaged homes left behind by the tornado, he said.
"We are transitioning now into the help stage," Judge said.