Belfast Crown Court heard Eddie Breen was arrested last February but remained silent during police interviews. He was later released without charge.
John Paul Wootton, 20, of Collindale, Lurgan and Brendan McConville, 40, of Glenholme Avenue, Craigavon, both deny murder.
Constable Carroll was shot dead in Craigavon in March 2009, after responding to a 999 call.
Tracking deviceOn Wednesday, a transcript of a police interview was read to the court which outlined the accusations put to Eddie Breen by a detective.
It included that he had planned the murder and fired the gun.
The court was also told that a partial print on the magazine of the AK47 gun used in the killing matched neither Mr Breen nor Mr McConville.
Meanwhile, a lawyer for Mr Wootton said his client was in his car in the area at the time of the killing, but was with a friend collecting a Chinese takeaway.
A police officer said that Mr Wootton had changed his statement about his movements which had been recorded on a tracking device hidden on his car by an Army intelligence unit.
The trial continues.
WITH MANY THANKS TO : BBC NEWS NI.
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