LA PORTE CITY — Scores of volunteers returned Wednesday for a fourth day as the search for an autistic teenager missing since Saturday night continued.But by the end of the day, La Porte City police Chief Chris Brecher said volunteers from the public would not be needed again Thursday, while Black Hawk County Sheriff Tony Thompson said the team of professional investigators has grown and is conducting a “robust” probe.“We are still treating this as a missing person,” the sheriff said Wednesday evening.For days, volunteers have been scouring fields, streams and wooded areas for Jake Wilson, 16.