The biggest murder mystery in history may be a step closer to solution.

The whodunit starts 250 million years ago, when 95 percent of all life in the sea was destroyed, and 70 percent of the life on land.

For decades scientists have been looking for evidence to finger the culprit. Some thought it might have been a massive meteor strike similar to the one that might have killed the dinosaurs, while others pointed to some geological process.

At the University of Calgary a team thinks it may have the 'smoking gun' that points to the latter. Where a policeman might find powder burns or residue to finger a shooter, these scientists found layers of coal ash in rocks from Canada's high Arctic.

There are only two possible sources of coal ash. One is burning it in a power plant. The other is from volcanoes - big ones. The layer of ash discovered in Canada is the first direct evidence of those volcanic eruptions.

Dr. Steve Grasby, adjunct professor in the University of Calgary's Department of Geoscience and research scientist at Natural Resources Canada, led the research team. He studied the formations with Dr. Benoit Beauchamp, a professor in the Department of Geoscience at the University of Calgary. Dr. Hamed Sanei, an adjunct professor at the University of Calgary helped with examining the peculiar carbon layers they had discovered.

The location of the volcanoes, known as the Siberian Traps, is now in northern Russia, centered on the Siberian city of Tura. They cover an area just under two million square kilometers, greater than that of Europe. The ash plumes from the volcanoes traveled to regions now in Canada's arctic where coal-ash layers were found.

We saw layers with abundant organic matter and Hamed immediately determined that they were layers of coal-ash, exactly like that produced by modern coal burning power plants, said Beauchamp in a statement. Sanei added that this discovery is the first direct evidence for coal ash during this extinction, which may not have been recognized before.

The coal ash wasn't the direct cause of the extinction. But it was one of several factors that finally pushed many ecosystems over the edge.

The ash, the authors suggest, may have caused even more trouble for a planet that was already heating up with its oceans starting to suffocate because of decreasing oxygen levels.

These eruptions would have sent significant CO2 gas into the atmosphere causing rapid global warming, Grasby said in an email. There would also be large ash clouds blocking the sun, as well as SO2 gas that would cause acid raid and impact the ozone layer.

On top of all that, the volcanoes erupted right through thick deposits of coal. The lava, Grasby said, would have burned it. That would create even more CO2, and exacerbate the warming. Coal also has a lot of toxic metals in it - it's a recurring problem for power plants, and requires scrubbers on the smokestacks. In this case, the burning coal sent fly-ash, which contains the metals, right into the ocean. The metals leavhed into the water, and Grasby's team found spikes of metals in the same layers as the ash. In an already very stresses earth, there was significant toxic metal release into the environment, he said.

The result, he adds, is that the Permian extinction wasn't a single event, but many stressors on the environment at once that finally pushed many life forms over the edge.