"Someone was cleaning up and left, then 10 minutes later saw smoke," Mayor Bruce Packer told Daily Voice.
No one was in the Holmes building when the blaze broke out just before 1:30 p.m. and no injuries were reported, he said.
The building is used for meetings and other events, including annual Girl Scout cookie dropoffs, parents said.
Firefighters had the three-alarm blaze under control in well under an hour.
Parents picked up their children at the church.
A Fair Lawn FAST responded, along with colleagues from Fair Lawn, Hawthorne, Ho-Ho-Kus, Midland Park and Ridgewood, while Allendale, Paramus and Wyckoff stood by at Glen Rock headquarteres.
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