Lab tests have concluded that 14 congregation members at Gustaf Adolph Lutheran Church in the small town of New Sweden were poisoned with a significant amount of arsenic in their snacks and coffee.
Investigators found a container at the church that had traces of arsenic.
"That would lead us to believe that these people were subjected to arsenic, and that it would have been in a consumable or ingestible item," said Lt. Dennis Appleton of the Maine state police.
An autopsy on the body of Walter Morrill, 78, who died after falling ill following the church gathering, was inconclusive, and toxicology tests were planned.
Appleton said police were treating Morrill's death as suspicious, "but only because we don't have the answers. We have no evidence it's criminal."
Five of those who became sick after Sunday's services were still too ill to talk with detectives; at least three were in critical condition.
The victims all shared coffee, sweets and sandwiches Sunday that were left over from a church bake sale the day before. Several church members said people who drank the coffee had complained that it tasted funny.
State Health Director Dora Anne Mills said Tuesday that arsenic, which can have a bitter taste, was the probable cause of the illnesses.
Appleton ruled out the coffee's sugar supply as the arsenic source and said the church's well was not contaminated. Arsenic is common in groundwater in Maine.
Life is safe and predictable in this remote section of northern Maine. The entire state averages about 20 murders a year, so small town speculation is swirling.
"My first thought is that it had to be an accident. I hope to God that it was nothing else," said Morrill's son, Ronald Morrill.
But this mystery won't go away. State police plan to interview everyone in the church, in search of anyone who might have a motive for murder.