A criminal investigation is under way into the deaths of two boys who were killed by a python that apparently escaped its aquarium in an apartment on top of a pet store, police said Monday.

Police arrived at the apartment in Campbellton, New Brunswick, at 6:30 a.m. and found the two boys, Noah Barthe, 4 and his brother Connor Barthe, 6, dead. A friend of the brothers was sleeping in another room and was unharmed, according to Police Constable Julie Rogers-Marsh.

Royal Canadian Mounted Police (RCMP) officials told the Canadian Broadcasting Corp. that the owner of the pet store, Jean-Claude Savoie, was hosting the two boys for a sleepover with his own son. Savoie reportedly discovered the bodies of the two boys and called the police. 

Rogers-Marsh said the snake, described as an African rock python, was believed to have made its way into the boys' room through the ventilation system overnight. Authorities had originally believed the python escaped from the pet store, but said Tuesday the snake was in an aquarium in the apartment. 

The snake was captured and killed by a veterinarian.. Calls to the pet store were not returned Monday. In New Brunswick, pet stores are allowed to sell non-venomous snakes of up to 3 meters, or just under 10 feet, in length. 

"It's very, very unusual and very tragic and difficult for everyone involved," Rogers-Marsh said.

Autopsies will be performed on the two victims on Tuesday.

The town's deputy mayor, Ian Comeau, said Reptile Ocean was licensed to operate and "everything was according to our bylaws, to the provincial guidelines."

The deaths of the boys have been "a shock ... it is unbelievable," Comeau said Monday evening. Comeau said he saw alligators, crocodiles and snakes when he toured the shop with the fire department about two years ago.

Lisa Janes, a co-owner and curator of Little Ray's Reptile Zoo, a private zoo and education program, said snakes don't usually see humans as food.

"We were absolutely shocked and saddened," she told the CBC. "Our condolences go out to the family."

Click for more from CBC.ca

The Associated Press contributed to this report.