LAVAL, Que. - The lawyer for a Quebec mother accused of killing her two daughters said Monday that 10 years after the girls were found dead in their playroom, there is still no explanation of what happened."The mystery remains whole," Pierre Poupart said during the first day of closing arguments. He told jurors there were riddles in the case they would be talking about long after the trial ended.His client, Adele Sorella, is accused of killing her daughters Amanda, 9, and Sabrina, 8. The cause of death was never established after the girls' bodies were found, dressed in their school uniforms, in the family home on March 30, 2009. There were no signs of violence on their bodies.The Crown suggested during the trial the sisters could have been killed in a hyperbaric chamber. The family bought the apparatus to treat Sabrina's juvenile arthritis. A pathologist who testified on behalf of the Crown identified the chamber as a possible cause of the deaths, suggesting the girls could have suffocated inside it.Poupart, however, reminded jurors that an expert in materials testified that he did not find any fibres from the girls' clothing on the mattress cover inside the hyperbaric chamber. The expert also did not find any fibres from the mattress cover on the girls' clothes."This is part of the mystery," Poupart said. The lawyer warned jurors against thinking Sorella could have planned the murders and thought about possible fibre transfers: "She is not a criminal genius."Poupart told jurors that investigators never checked for fingerprints or signs of a break-in and failed to examine the hyperbaric chamber.Since the start of the investigation, there was a kind of "obsession of absolute certainty" that the person responsible for the girls' deaths was Sorella, Poupart said. The investigators' tunnel vision prevented them from looking for other clues in the case, he added.The Crown has maintained that only Sorella would have been able to cause the girls' deaths because no one else was in the home. But the defence has offered other theories.It brought up the fact Sorella's husband, Giuseppe De Vito, had installed a sophisticated alarm system in the home. De Vito, who died in prison in 2013, wasn't living with the family when his daughters were found dead. He was being sought by police as a suspect in an organized crime investigation."Ask yourselves this question: Could it be interpreted as a sign of worry that her husband installed an alarm system like that"The trial began in November. Poupart's closing arguments are scheduled to continue Tuesday.