A jogger saw the suspect in the vicinity of one of the torched churches and held him there while she waited for authorities to arrive, the Denver Post reported.
No injuries were reported in the Wednesday fires in south Denver.
"Our alert and aggressive jogger who held this guy, if she had not been there it could have been much worse," Denver City Councilman Charlie Brown said.
The first fire at the Rocky Mountain Miracle Center was put out in about 15 minutes and the second blaze, hours later, at the Church In Denver was extinguished in about 30 minutes, the Rocky Mountain News reported.
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