Fire officials said they were able to quickly determine it was arson because of a security camera facing the front yard.
In the security camera footage, Sanford fire officials said two people can be seen appearing to light something right outside of the two homes and what happened next concerned them.
"They disappear from the screen for a little bit and then there's a big flash. And these two people run away," Sanford Fire Chief Steve Benotti said.
Benotti said the flames appeared too quickly for it to be an accident.
"I wouldn't speculate on what was used but it does lend itself to say that it's probably some sort of accelerant, or whatever was used, to make the fire get that big that quick," Benotti said.
Investigators said they are now looking for those two people of interest and are asking for the public's help.
"If anybody was out or awake, if anybody has any cameras on their homes and they saw someone walking by, we would like to see that also," Sgt. Joel Davis of the state fire marshal's office said.
Benotti said no one was hurt but eight people are without a home.
"Twenty-six Montreal Street is a total loss. That building is completely gutted. It was not a building that was in good shape, to begin with," he said.
He said he believes the search for the suspect will be over soon.
"Pretty confident that we will get a person or persons involved and put them where they need to be," he said.