A five-month investigation into the cause of death of the last remaining beluga at the Vancouver Aquarium has concluded that the whales died from a toxin, but its source is still unknown.

A 21-year-old whale named "Qila" died suddenly at the Stanley Park facility where she'd spent her entire life on Nov. 16, 2016. The day after the white whale's death, her 29-year-old mother, Aurora, started showing similar symptoms. She died a week later.

The aquarium launched a months-long investigation into what killed the whales, a study that brought in dozens of veterinary pathologists, toxicologists, genome specialists, medical doctors and field researchers.

Unable to rule out whether a human had tampered with the whales or their tank, the aquarium said it had also contacted police about the deaths, though officers said no formal investigation had been launched. 

On Wednesday, aquarium announced that its investigation was over, and that the cause of deaths of both cetaceans was some kind of toxin.

"Extensive testing was unable to identify the exact substance involved, which is not uncommon due to the very limited time a toxin is traceable in the bloodstream," staff said in a statement.

"The investigation also determined that the toxin was likely introduced by food, water, or through human interference."

The aquarium's head veterinarian, Martin Haulena, said that the investigation into the "devastating" loss helped staff make several improvements to ensure the safety and welfare of marine mammals.

Since the deaths of Qila and Aurora, the aquarium is working on reducing the risk of toxins being consumed by closely screening food and all vegetation located near its habitats. To keep toxins from being ingested through water, its mechanical water treatment systems are being replaced as a precaution in all cetacean habitats. The facility is currently home to a false killer whale, a Pacific white-sided dolphin and a harbor porpoise.

To reduce the chances of a toxin being introduced by human interference, the facility has made "significant security updates." Staff said the updates were made to monitor perimeter access, but did not specify what changes had been made.

Speaking for the facility, Haulena said staff "deeply appreciate" the help of all of those who were involved in the five-month investigation and the improvements. Haulena also acknowledged the work of staff and volunteers, and thanked the public for an outpouring of support during what was a difficult time for those who worked closely with Qila and Aurora.

"They were beloved members of our family and the community for more than two decades. Their loss is felt profoundly by our staff, members, supporters, and the public," Haulena said.

The deaths sparked heated debates in the city about whether large marine mammals like whales and dolphins should be kept in captivity and put on display. The arguments culminated in a vote by the Vancouver Park Board in March to move toward banning the importation or display of cetaceans at the attraction.  The board was asked to present a draft bylaw amendment by May 15.

Park board commissioners said they anticipated the decision would be challenged in court, but felt the decision was within their mandate. The aquarium is on federal land but falls under the jurisdiction of the board, and is on a 60-year lease.

Earlier this year, the aquarium said it planned to open a new beluga exhibit with between three and five non-mating whales by the spring of 2019. The public display would then close by 2029 – the year its lease will be up for renewal – but it would continue to be home for whales that cannot be released into the wild.