Question: Warning: Spoiler. At the end of the movie, it's found that three of the victims had managed to escape; only one was actually returned to the parents. It isn't known if the other two were recaptured by the killer or managed to get away (but went into hiding). Why didn't the police just go back and question the killer's accomplice? He'd likely be able to reveal whether any of the boys had been recaptured. (We're never told if the accomplice had died by that point, or whether he was unavailable, or whether nobody bothered asking him.)
Answer: According to Whittier Daily News - Sanford Clark, the 15-year-old nephew/accomplice to Gordon Northcott... He served in World War II, married, and worked 28 years for the Canadian postal service. He and his wife, June, adopted and raised two sons. The couple were married for 55 years and were involved in different organizations. Clark died in 1991. Sanford was able to put the Wineville Murders behind him. I saw a book on Amazon about Sanford Clark's life after it was exposed. It's titled "The Road out of Hell," which might be really interesting.
Answer: When the accomplice is originally questioned he says that killer told him a few 'may have escaped', implying he never knew if they were caught.
Also, it's implied his accomplice a young boy himself was scared of ending up like the boys himself, sadly I think it was done for survival.