Task: Question Answering
Input (English): Passage: During the 1980s, there was increasing pressure on both the Polish and Soviet governments to release documents related to the massacre. Polish academics tried to include Katyn in the agenda of the 1987 joint Polish-Soviet commission to investigate censored episodes of the Polish-Russian history. In 1989, Soviet scholars revealed Joseph Stalin had indeed ordered the massacre, and in 1990 Mikhail Gorbachev admitted the NKVD had executed the Poles and confirmed two other burial sites similar to the site at Katyn: Mednoye and Piatykhatky.
On 30 October 1989, Gorbachev allowed a delegation of several hundred Poles, organized by the Polish association Families of Katyń Victims, to visit the Katyn memorial. This group included former U.S. national security advisor Zbigniew Brzezinski. A mass was held and banners hailing the Solidarity movement were laid. One mourner affixed a sign reading "NKVD" on the memorial, covering the word "Nazis" in the inscription such that it read "In memory of Polish officers killed by the NKVD in 1941." Several visitors scaled the fence of a nearby KGB compound and left burning candles on the grounds. Brzezinski commented:
It isn't a personal pain which has brought me here, as is the case in the majority of these people, but rather recognition of the symbolic nature of Katyń. Russians and Poles, tortured to death, lie here together. It seems very important to me that the truth should be spoken about what took place, for only with the truth can the new Soviet leadership distance itself from the crimes of Stalin and the NKVD. Only the truth can serve as the basis of true friendship between the Soviet and the Polish peoples. The truth will make a path for itself. I am convinced of this by the very fact that I was able to travel here.
Brzezinski further stated:
The fact that the Soviet government has enabled me to be here—and the Soviets know my views—is symbolic of the breach with Stalinism that perestroika represents.
His remarks were given extensive coverage on Soviet television. At the ceremony he placed a bouquet of red roses bearing a handwritten message penned in both Polish and English: "For the victims of Stalin and the NKVD. Zbigniew Brzezinski".On 13 April 1990, the forty-seventh anniversary of the discovery of the mass graves, the USSR formally expressed "profound regret" and admitted Soviet secret police responsibility. The day was declared a worldwide Katyn Memorial Day (Polish: Światowy Dzień Pamięci Ofiar Katynia). 
Question: What group of people were initially blamed for the massacre that was ordered to be committed in Katyn?
Output (English): Nazis.