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The Philadelphia Zoo’s five meerkats have died after they were exposed to a toxin.

The animals were exposed to the unknown substance as a topical agricultural dye was being applied, NBC News reported.

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The dye, Nyanzol-D, is frequently used to differentiate the animals from each other. It is used on several species at the zoo, not just the meerkats.

Sibling mob Nkosi, Lula(ma), Kgala, (Sethu)Nya and Ari arrived at the zoo in 2014, joining the zoo’s previous meerkat Pete.

The dye was used on the animals on June 1 and one was found dead about 30 minutes later. The others started showing signs of illness. Veterinarians pumped their stomachs, but three of the meerkats died within 36 hours. The last one died Monday, Fortune reported.

Zoo officials told WTXF that the dye, which comes in markers, has been used for 20 years at the facility. Vice President of Animal Well-Being Rachel Metz told the television station she suspects something else happened.

Officials at the Philadelphia Zoo have stopped the use of the dye until they can find out what killed the meerkats, WPVI reported. Results from the necropsy are pending.

The animals’ deaths are also being investigated by the Association of Zoos and Aquariums and the U.S. Department of Agriculture, WTFX reported.