It's been a difficult time for the London Zoo.

The London Zoo announced on Friday that a new male Sumatran tiger, Asim, had killed the zoo's prized female Sumatran tiger, Melati.

Melati died "while being introduced to new male Asim for the first time," the zoo announced in a tweet. The two tigers were allowed to interact in the hope that they would eventually mate as part of a European breeding program. The International Union for the Conservation of Nature (IUCN) lists Sumatran tigers as "critically endangered."

Tiger introduction is "high risk," the zoo said, so what happened to Melati was a realistic possibility.

Many of Melati's fans turned to social media to mourn her death.

The London Zoo was also criticized by individuals who would like to see zoos abolished.

The introduction was carefully planned, as Kathryn England, chief operating officer of the London Zoo, noted in a blog post:

After careful observation, we all felt confident that the timing was right to introduce Asim to Melati. With more than 120 years of collective experience managing tigers between us, even with the benefit of hindsight I am confident we'd all make the same decision again based on the behaviour observed. Many of us at the zoo had seen them greeting, chuffing and sniffing each other, with no behaviour that caused concern. Several people from outside the zoo have remarked that 10 days seems fast to introduce tigers to each other. It's not – it's wildly variable and depends entirely on careful observation of their behaviour. Conversely, it can be risky to leave tigers showing an interest in each other out of contact for too long, leading to a build-up of tension and frustration.

In the lead-up to Friday, there was a lot to do. Careful plans had been meticulously drawn up, and risk assessments completed. Every detail had been discussed and examined at length amongst the team, who on the morning itself gathered at Tiger Territory to allow Melati, 10, and Asim, 7, to meet in the flesh for the first time.

Big cat introductions "always carry risk," she wrote:

The team were not empty-handed; big cat introductions tend to be highly charged and always carry risk. Fire extinguishers, airhorns, hoses and flares were at the ready to distract them if the encounter took a dangerous turn. We had tranquiliser darts on hand but there are actually few scenarios where this would be viable – even if you could be sure of hitting the right tiger, they take up to 30 minutes to take effect. This could be even longer in a high-energy situation when an animal's adrenaline is flowing. All of which means using one would have had no impact on the outcome.

The perimeter of the exhibit was carefully walked and checked for anything out of place that could alarm the tigers. The surroundings were quiet and calm. We opened the gate between the two enclosures and watched carefully. Initially everything about their meeting was as we expected. They watched each other, they were cautiously interested and they sniffed the air for several minutes.

At Tiger Territory we have created a space where our tigers can exhibit their natural behaviours. There are places to hide, places to be out of sight, places to be up high. Our tigers can behave like tigers, which are the conditions a good and responsible zoo is looking to create for all the species it holds.

However, this meeting did not go the way England and the other staff members if the London zoo hoped.

This is truly a tragedy for animal lovers worldwide.