The Saturday afternoon blaze caused severe damage to the Beaverton Freewill Baptist Church in northwest Alabama, near the Mississippi line.
"It's definitely arson," said Ragan Ingram, a spokesman for the state fire marshal's office. It was not immediately clear if the fire was connected to the other blazes that have destroyed or damaged nine churches since Feb. 2.
Saturday's fire was the only one that wasn't set in the pre-dawn hours.
Investigators have said they do not know a motive, but there is no racial pattern. Five of the churches, including the one in Beaverton, had white congregations and five black. All were Baptist, the dominant faith in the region, and mostly in isolated country settings.
Last week, Gov. Bob Riley said the nine earlier church fires appear linked, as investigators checked out witness reports of two men in a sport utility vehicle seen near a number of the fires.
On Sunday, a federal investigator said authorities don't have specific suspects in mind, but they are seeking two white men in their 20s or 30s. Eric Kehn, a spokesman for the federal Alcohol, Tobacco, Firearms and Explosives agency, said the assessment was based on witness reports and behavioral profilers.
"They're not youths or teens. It's probably someone in their 20s or 30s. We believe they're pretty much inseparable. They're something like bosom buddies," he said.
The FBI has been looking into whether the arsons were civil rights violations under laws covering attacks on religious property.
Evidence from one of the earlier fires indicates the perpetrators may have been briefly trapped inside the building and may have been hurt, said Jim Cavanaugh, ATF regional director.