A reptile expert in the Maritimes who worked directly with the python linked to killing two boys in New Brunswick says the large snake had a "temperamental disposition" and wasn't used to being around humans.

RCMP have launched a criminal investigation into the deaths of Noah Barthe, 4, and Connor Barthe, who would have celebrated his seventh birthday in just a few weeks.

Maritime Reptile Zoo Curator Mike MacDonald said the snake was in excellent condition when he handled it six weeks ago, but it was known for being aggressive.

He said they were cautious when entering its enclosure.

"It is very protective. It’s very defensive. It's not one that's regularly around people," he said. "It was extremely nervous with people."

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Maritime Reptile Zoo curator Mike MacDonald said the python (not pictured) was in excellent condition when he handled it six weeks ago. (CBC)

"It's extremely rare. It's rare that they would kill one person let alone two people at one time."

Initial reports suggested that the snake escaped from the store and made its way through the ventilation system into the apartment above, where the brothers were on a sleepover.

On Tuesday, RCMP Sgt. Alain Tremblay said that the snake was actually held in a glass enclosure on the second floor. It is believed it escaped through the top of its cage into the ventilation system above. The snake eventually came through the ceiling and down into the living room, where the brothers were sleeping.

Reptile Ocean owner Jean-Claude Savoie was hosting the two boys for a sleepover with his own son. Savoie reportedly found the two boys and called 911. His son was in another room and was unharmed, police said.

MacDonald said he wouldn't comment on the facilities since they are under investigation.

The snake was put down on Monday.

Steven Benteau, a spokesman for the Department of Natural Resources, said African rock pythons are not allowed in New Brunswick under the Exotic Wildlife Regulation.