The President of Poland, Lech Kaczynski, and other high government officials have died in a plane crash in Smolensk, Russia, on the way to a commemoration of the Katyn massacre (NYT, WaPo, LA Times, Guardian).
The parallel is horrifying. During World War II, more than twenty thousand Polish military officers were murdered at Katyn, by Russians, in order to destroy possible post-war Polish leaders and ensure Soviet dominance in Poland after the war. Now much of the Polish government has been destroyed in a plane crash.
Although the Russian government has not quite apologized for the massacre, Vladimir Putin was planning to join the Polish president at a commemoration of the event in a gesture of reconciliation. Putin has assumed charge of the investigation into the crash.
The Polish Prime Minister, Donald Tusk, was not on the plane and is the head of government, so this should not provoke a governmental crisis.
There will be rumors and suspicions in Poland, since the crash took place in Russia. One may argue that with symbolism of this kind, the Russians would be crazy to do such a thing, but many in Poland find the Russians capable of any evil you can imagine and more.
Update: James Fallows gives a plausible scenario for how the accident may have happened. It probably wasn't the plane.