The man, aged 71, was arrested in Omagh on Saturday evening under Section 41 of the Terrorism Act.
He has been to Musgrave Serious Crime Suite for questioning by MIT detectives.
DCI Caldwell was shot up to ten times on Wednesday, February 22, at the Killyclogher Road area of Omagh.
The senior detective had been training young players from his club Beragh Swifts on football pitches.
As he was putting footballs into the boot of his car, two masked men came out of the shadows and opened fire on him.
DCI Caldwell’s condition is described as “critical and sedated” in Derry’s Altnagelvin Hospital.
Five men aged 22, 38, 43, 45 and 47 years old, also arrested in connection with the attempted murder, remain in police custody at this time.
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