A 13-year-old New Mexico boy, whose body was recently found in a shallow grave, was allegedly abused and forced to spend his last days in a dog kennel, authorities said.

Investigators claim Jeremiah Valencia was tortured by his mother's boyfriend, Thomas Ferguson, for years. According to Santa Fe County Sheriff's Facebook page, Jeremiah died after being "subjected to horrifying circumstances".

"He was placed in a dog kennel for hours on end without food. It's absolutely heart breaking, these acts are senseless," District Attorney Marco Serna said.

"I can honestly say that this poor child suffered at the hands of a monster," Santa Fe County Sheriff Robert Garcia said at a press conference on Tuesday (30 January).

"It's hard to sleep at night just thinking about the abuse this poor child went through," Garcia was quoted as saying by KRQE television channel.

Three people — Jeremiah's mother, Tracy Ann Pena, 35, Thomas Wayne Ferguson, 42, and Ferguson's son Jordan Anthony Nunez, 19 — were arrested in connection with the case earlier this week, Fox17 Online reported. They have been charged with abuse of a child resulting in death, tampering with evidence and conspiracy to commit tampering with evidence and are being held without bond.

New Mexico State Police Chief Pete Kassetas said that Ferguson was "the main suspect" in the case.

Ferguson is also said to have a long criminal past. In 2014, he was charged with rape, kidnapping, and domestic violence involving another woman.

Authorities are also investigating if the teenager was sexually abused in any way. The boy's body was discovered along New Mexico State Road 503 near Nambe. According to Garcia, the boy had been missing for months but nobody tried to report his disappearance.

The issue came to light after the boy's mother who was arrested in another case told an inmate about her missing son.

The inmate in return alerted the deputies about what the woman had shared, which sparked an investigation, KRQE reported.