Up to six small fires were touched off at four churches in Mesa, Arizona, since mid-May by burning Bibles. The most recent fire was set on July 7 at Calvary Baptist Church. (Facebook)

Arizona authorities are probing a series of arson fires at suburban Phoenix churches as possible hate crimes after an apparent arsonist set Bibles alight on church doorsteps, police said on Thursday.

Authorities said up to six small fires were touched off at four churches in Mesa, Arizona, since mid-May by burn