The man, aged 45, was detained in Belfast under the Terrorism Act on Tuesday evening.
Det Ch Insp John Caldwell suffered life-changing injuries in the attack in Omagh last month.
Investigators said their main line of enquiry into the attempted murder was dissident republican group the New IRA.
Eight other men, aged 72, 57, 47, 45, 43, 33 and 22, arrested in connection with the attempted murder have since been released.
A 25-year-old man, detained over a claim of responsibility made for the shooting was also released following questioning.
Last week, the independent charity Crimestoppers announced a new reward for information about the attack.
It had previously offered £20,000 but said anonymous donors had helped to increase that to £150,000.
Det Ch Supt Eamonn Corrigan said the Police Service of Northern Ireland (PSNI) was "working relentlessly to bring those responsible to justice".
Det Ch Insp Caldwell, 48, was left critically ill after he was shot while putting footballs into his car after coaching young people at football.
The shooting, which occurred in front of school children, has been widely condemned by political figures across Northern Ireland.
He is one of the best-known detectives in the PSNI, often fronting press conferences on major inquiries during his 26-year career.
Mr Caldwell had coached a Beragh Swifts training session at Youth Sport Omagh when the gunmen approached and shot him at about 20:00 GMT on 22 February.