It’s rare for police agencies to publicly acknowledge investigative blunders like Boston’s top cop did after the revelation detectives failed to arrest a man for a violent Roxbury assault last September who is now charged in two Southie attacks and is also a “person of interest” in the Amy Lord murder, experts said yesterday.

“Most police departments are much more defensive,” Jack Levin, a Northeastern University criminologist, said. “It’s unlikely departments would admit making such an important mistake permitting a hardened criminal with a lengthy record of violence to remain on the streets.”