Mansfield fire investigators were on the scene Wednesday of the city's 22nd suspicious fire since mid-June.
The latest blaze followed three other fires that were started overnight.
The most recent fire was started in a mattress that was intentionally propped up against the wall of a shed.
"They are definitely not all related, some of them are in the same neighborhood - we think maybe. Others are not in the same neighborhood, and we have some people that we're looking at pretty seriously about a few of those," said Fire Chief John Harsch.
Most of the fires have started in vacant houses, but firefighters said arsonists are also targeting porches and garages.
According to investigators, the only thing linking the three overnight fires is the time frame.
A garage fire broke out after 6 p.m. on South Foster Street, a vacant house on West First Street caught fire around 7:30 p.m., and a porch of a home on South Adams Street was set ablaze around 11 p.m.
Firefighters said they have not determined what caused the four latest fires or even if they are definitely related.
"And then when you have more than one in a night, and three in a night, that really taxes the firefighters physically, because they're just spent," added Harsch.
While officials are working to get ahead of the flames, it seems they are just a step behind who is starting them.
No neighboring homes were damaged in last night's fires and no one was hurt.
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