CAMPBELLTON, N.B. -- A 45-kilogram python lunged, snapped its jaws, and made "growling noises" when it was forced back into its pen after killing two sleeping boys, an RCMP officer told a New Brunswick jury Tuesday.

Const. Stephane Dugas described the scene on the morning of Aug. 5, 2013, shortly after Jean-Claude Savoie, who is facing trial for criminal negligence in the boys' deaths, called 911.

Dugas testified that he found Savoie wearing a bloody shirt, two boys who were beyond medical help, and a 4.7-metre snake in the laundry room.

Savoie followed Dugas' instructions to return the snake to its enclosure, where it made noises, rose up almost 1.8 metres and lunged at the glass, Dugas said.

The two boys -- four-year-old Noah Barthe and his six-year-old brother, Connor -- were covered in red marks, and one had a lot of wounds.

"I knew at the time not much could be done," said Dugas. "There was lots of blood."

He said a paramedic examined the boys, but just shook her head.

Savoie, who now lives near Montreal, owned the Reptile Ocean pet store below his Campbellton apartment. The boys were having a sleepover with his son.

Crown attorney Pierre Roussel told the jury the python likely used an air duct to escape its enclosure in the apartment -- and had tried to escape before.

"Mr. Savoie committed a breach of duty to take care of those children when they were left with him by their mother," Roussel told the jury. "He didn't kill them himself but he failed to take precautions."

Roussel said in his opening statement they will hear the snake had gotten into the duct before, and that an employee had warned Savoie the vent cover needed to be repaired.

"Mr. Savoie neglected to cover said vent and left an opening in his snake pen and that through that the snake was able to escape and cross through the vent and drop into the living room."

He said the vent cover was found on the floor of the snake enclosure.

"Mr. Savoie was aware of the behaviour and nature of this African rock python that he was keeping. You will hear evidence that this snake was pretty aggressive. That there were special measures that had to be taken in order to care for it," he said.

Roussel called the deaths of the boys "a sad story, a tragic story." Savoie was a family friend who had taken the boys shopping and to a farm before the sleepover with his son.

Some people in the public gallery became emotional as the audio of Savoie's 911 call was played for the jury Tuesday.

"Two kids are dead," Savoie told the dispatcher. "There's a python that got out and killed the two kids."

He told the dispatcher that the snake was still loose, and he had to go back upstairs because he had another child up there.

"Can you just send someone here please?" Savoie said to the dispatcher.

Another police officer who responded to the scene also told the court Tuesday that he observed the python being very aggressive after it was put back into its enclosure.

"It was standing on its tail and charging the glass," Sgt. Rene Labbe said.

"I observed the snake trying to reach the vent hole in the ceiling."

Labbe said Savoie then grabbed the snake and put it in a garbage bin. It was then padlocked and removed from the property.

As the trial got underway Tuesday, the judge instructed the four-man, eight-woman jury about the presumption of innocence, and said Savoie starts with a "clean slate" at the beginning of the trial.