When he arrived — luckily at low tide — he found the animal, 50lb male pit bull mix, with a thick chain around his neck affixed to an exposed steel rebar rod protruding from a heavy rock, hidden away on a secluded part of the beach.
The dog had no way of escaping as the tide slowly approached — and the watermark was well above his head.
"During certain parts of the day this portion of the beach is completely submerged, with the tide rising as high as street level, past the point where the dog was chained," Massachusetts State Police wrote in a Facebook post. "The dog was at risk drowning in a couple of hours if he hadn't been found."
Determined to find the cruel owner, Trooper Mastromattei began a five-day investigation — much of it working on his own time — until he tracked him down.
He canvassed the area for residential surveillance video and found four showing a Hispanic male with facial hair walking the dog toward the beach — one even showing him making his way down the beach ramp to the sand with the oblivious pooch.
In all four videos he carried a metal chain collar and leash matching the one pinning the dog to his doom.
Officers eventually tracked Pacheco-Osario to his construction site workplace, where they confronted him; he admitted walking the dog to the beach, tying him up and abandoning him. He told them he "decided he could not keep the dog any longer."
He also told the troopers he had named the dog, somewhat ironically, "Killer."
Pacheco-Osario was arrested and charged with animal cruelty. He was released on $340 bail.
Speaking to reporters outside his court arraignment, he apologized for what he'd done, claiming a friend in Virginia had given him the dog just six months ago, but he was having issues with him and couldn't get a shelter to take him, CBSN Boston reported.
He is due back in court November 3.
"Killer" meanwhile is in good health, and has since been renamed Carmelo. He is being cared for at Ocean View Kennels until they can find him a new, and hopefully slightly more appreciative home.
"His canvas is young so his canvas is still pretty blank so now he can just make his own beautiful story," one of his carers told the outlet.