A high-ranking law enforcement source told the news station that while the fire has not been determined as arson yet, it is being investigated as one.
"We are working on the belief that this fire was set," the source said.
Firefighter Jon Davies was killed in the Dec. 8 blaze after he and his partner, Firefighter Brian Carroll, became trapped in a burning triple-decker when it collapsed on them.
They went back into the building after being told there was someone still inside, but it ended up being a false report.
No bodies were found in the rubble of the fire, officials announced Monday, and investigators are trying to find the resident who was believed to be trapped.
"It's a waste of life, but again, it is our job," Worcester Deputy Fire Chief Geoffrey Gardell told the news station. "We want to make sure that there isn't any sole survivor in that building and we're going to go in there and check for that."
Davies' family told the news station Tuesday that they support the decision made by the department during the fire.
"We have no doubt in our minds they made the best decision possible with the information that they had available to them on that fateful morning," Rob Davies, Jon's brother, said.
Thousands are expected to attend the fallen firefighter's funeral on Thursday.
