Short answers, pulled from the story.
Six members of the Soviet Politburo signed the execution order including Joseph Stalin, Vyacheslav Molotov, Lazar Kaganovich, Kliment Voroshilov, Anastas Mikoyan, and Mikhail Kalinin. The document targeted 25,700 Polish nationalists and counterrevolutionaries held in camps and prisons.
The executions began after the 3rd of April 1940 and a total of 21,857 Polish internees and prisoners were executed according to declassified documents from 1990. Of these victims 14,552 were prisoners of war from three main camps and another 7,305 were held in prisons in western Byelorussia and Ukraine.
Mikhail Gorbachev admitted that the NKVD had executed the Poles on the 13th of April 1990. On this same date the USSR formally expressed profound regret and admitted secret police responsibility for the crime.
About 4,421 victims came from Kozelsk camp while another 3,820 came from Starobelsk and six thousand three hundred eleven came from Ostashkov. These three locations supplied the bulk of the 14,552 prisoners of war who were executed.
President Franklin D. Roosevelt rejected a report by Navy Lieutenant Commander George Earle concluding Soviet guilt and ordered the document suppressed to appease Stalin during the war against Nazi Germany. Washington kept information about proof implicating the Soviets secret until documents released in September 2012 revealed coded messages from U.S. POWs Capt. Donald B. Stewart and Col. John H. Van Vliet Jr.