Sgt. Dennis A. Woodring of the Harrisburg, Pa., Police Department brought a dog trained to detect accelerants, but the animal found no clues.
The investigators spent most of the day near the rear entrance of the brick church. Photos taken by the McConnellsburg Fire Department at the height of the blaze showed the most intense flames at the rear of the building.
The church was empty when the fire erupted around 4:30 a.m. Saturday.
The ATF has assisted at all church fire investigations since a series of arsons destroyed or damaged a several Southern black churches last year.
So far this year, seven rural churches in western Pennsylvania have been burned by suspected arsonists, according to ATF spokesman Steve Haskins. Like Calvary Baptist, the other churches have mostly white congregations.
Investigators said they could not rule out arson in the Fulton County church fire, but have established no connection between all of the church blazes.
"I hope this doesn't turn out to be arson," said the Rev. David M. Carroll, pastor.
"If it is, who will be next?" said Edgar Lake, one of the first deacons of the church. The congregation was organized in 1962. The church was built in 1970.
Lake's wife, Gladys, said she was in bed when a friend from McConnellsburg called shortly before 5 a.m. Saturday to say that she heard on the police monitor that there was a structure fire in the vicinity of the church.
"A few minutes later, a member of the church called and said it was fully involved," she said. "I'm just glad I wasn't here to see it burn."
Gladys Lake spent most of the day sitting in her husband's pickup truck in the church parking lot trying to separate and dry out precious photographs of the congregation taken over the years at church functions. The album was pulled out of the water-soaked basement.
Carroll said area churches have been calling with offers of help.
"A church in town is giving us 80 hymnals and another congregation has offered us the use of their church for our services," he said.
Carroll said his 100-member congregation will hold Sunday services in the Fulton Theater Welcome Center for now. The congregation may also upgrade a large classroom building it owns next to the burned-out church as a temporary meeting place until the church is rebuilt.
Fire department officials estimate the damage to the church and its contents at $375,000. Carroll said insurance on the building would cover that much.