News of the killing shocked the racing world. Scott, 34, regarded as one of the brightest young trainers, had 90 horses, most of which belonged to Sheikh Maktoum al-Maktoum.
The man, Clem O'Brien, the former stud groom at Scott's stables, was being held by detectives at Parkside police station in Cambridge.
Mr Scott died of a single shotgun blast in the chest just after 6pm on Friday. His body was found in the barn of his estate. He had been clubbed over the head and shot.
Armed police launched a search of the area and called in the Essex police helicopter to assist. A shotgun was found in Cheveley on Friday night and yesterday morning Mr O'Brien was picked up by police on open land near the village.
Detective Superintendent Alan Fletcher said yesterday he was not looking for anyone else: 'We have found a firearm.'
Scott, a trainer since 1989, enjoyed a brilliant career. He had trained more than 100 winners, including the 1991 Breeders Cup Sprint winner, Sheikh Albadou, and achieved two classic wins, both with Possessive Dancer, which won the 1991 Irish and Italian Oaks.
There was a minute's silence at the race meeting yesterday in nearby Newmarket.
Scott's wife, Julia, and children - Daniel, 7, Emily, 4, and Rupert, 2, - were at the house when the shooting took place. Yesterday they were being comforted by friends and relatives.
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