God sent an evil spirit to Saul (?)

I am reading the account of Saul in I Samuel and was somewhat puzzled when I read that 'the spirit of the Lord left Saul' and then 'God sent an evil spirit to Saul'.

I am reading the bible, cover to cover, for the first time - I have never heard reference to God sending anyone an evil spirit. Is that the only time that occured? I though Satan sent evil spirits and God himself could do no evil. Your insights would be helpful.

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