Most of the churches affected had predominantly African-American congregations.
"We believe that this fire-setting activity is meant to send a message," said John Ham of the Bureau of Alcohol, Tobacco, Firearms and Explosives.
The most recent fire broke out early on Thursday morning.
"It was really disheartening to see what you worked so hard for brought down in a matter of minutes," said Pastor David Triggs of the United Believers in Christ Church, which was heavily damaged on 17 October.
The fires have all been set at the front or side doors of buildings, usually without causing serious structural damage.
This was not the case for Pastor Triggs, who said his church's building is likely unsalvageable.
That leaves his roughly 70 active congregants without a house of worship.
He is holding services on the lawn this coming Sunday while trying to determine next steps.
"We're kind of at a cross road," he said. "I have no doubt that God is going to provide for us."
Police in the US state of Missouri are searching for arsonist who set fire to seven churches in the St Louis area in the past two weeks.