Karowei is also alleged to have led the gang responsible for the beheading and killing of a Nigerian Navy Captain in Toru-Ndoro community, Ekeremor council area of Bayelsa State.
He was said to have been arrested on Thursday.
The kidnap kingpin, who had been declared wanted, was alleged to be the brain behind the killing of Ian Squire, 57, seized by the militant gang during the October 13 abduction of the Christian missionaries.
Military said the wanted militant had voluntarily surrendered himself to the military at Ogbogbagbene community in Burutu council area of the state.
Although details of his arrest were still sketchy at the time of filing this report, community sources confirmed the development to this correspondent.