So my position on this right now is that the murders may have all been done by Berkowitz, but this in no way rules out larger and more involved collective of others participating in these crimes.
Let's just run through a couple avenues as SOP (standard operating proceedure) in matters like this one. First employ some anagrams. Berkowitz used a Charter Arm's 44 special to commit the murders.
An anagram for special is Plaice.
Plaice is a flat bodied fish eaten as food.
Anagram for Berkowitz is: Be Zit Work
*8 letter Anagram for Berkowitz is Reitboks: any of several reedbucks (especially Redunca arundinum) of southern Africa having a bushy tail and in the male small ringed horns that curve forward. Antelopes from the genus Redunca.
The Dog's name was Harvey. Anagrams for Harvey: Ah Very or Ha Very
The name Harvey grabbed my attention because of it's association to film involving an invisible giant rabbit.
Harvey is a 1950 comedy-drama film based on Mary Chase's play of the same name, directed by Henry Koster, and starring James Stewart and Josephine Hull. The story is about a man whose best friend is a pooka named Harvey – in the form of a six-foot, three-and-a-half-inch tall invisible rabbit.
A pooka is a mischievous or malignant goblin or specter held in Irish folklore to appear in the form of a horse and to haunt bogs and marshes.
James Stewart plays a character named Elwood drinks and might also be mentally ill.
Elwood is another repetitive name in film. An anagram for Elwood is dewool; to remove the wool.
"A Dr. Sanderson convinces Elwood to come into his office where he will receive a serum called Formula 977 that will stop Elwood Dowd from "seeing the rabbit". As they are preparing for the injection, Elwood's sister is told by their cab driver about all the other people he has driven to the sanatorium to receive the same medicine, warning her that Elwood will become "just a normal human being. And you know what stinkers they are."
She goes back an stops the doctor from injecting Elwood with the serum.
"As Elwood is leaving, A Dr. Chumley asks Elwood for Harvey's help, and Elwood, being the obliging fellow he is, makes no objection. Dr. Chumley, arm in arm with an invisible companion, asks "Have you ever been to Akron?"
"In the final scene of the film, Elwood (along with everybody else) arrives back at the hospital. By this point, Dr. Chumley is not only convinced of Harvey's existence, but has begun spending time with him on his own, with a mixture of admiration and fear."
Well...that provided some valuable insights.