Uhh Gorbachev admitted it in 1990.
Anyway, here's a translation of the memo Beria sent to Stalin regarding the situation with Polish POWs and political prisoners. Rounded parentheses are Beria's, square brackets are my notes.
"People's Commission of Internal Affairs [NKVD]
March 1940
No. 794 B
"To the Central Committee of the All-Union Communist Party (Bolsheviks),
To Comrade Stalin
"In the prison camps of the NKVD of the USSR and in the reserve prisons of Ukrainian and Belorussian oblasts [small administrative district] are currently contained a large quantity of former officers of the Polish Army, former employees of the Polish police and intelligence agencies, members of the Polish nationalist counterrevolutionary party, uncovered participants of counterrevolutionary rebel organizations, defectors and others. All of them are sworn enemies of the Soviet state and full of hatred for the Soviet system.
"POW officers and policemen, while in the camps, are attempting to continue counter-rev. work and lead anti-Soviet agitation. Each of them is just waiting to be released in order to actively join the opposition to the Soviet state.
"Organs of the NKVD in the western oblasts of Ukraine and Belarus revealed a number of rebel counter-rev. organizations wherein an active leadership role in being played by former officers of the Polish Army, former policemen, and gendarmes.
"Among the deserters arrested for illegal border crossings, there was found a significant number of persons whom are members of counter-rev. spy and insurgent organizations.
"In the POW camps there is a total of (not counting enlisted soldiers and non-commissioned officers) 14,736 former officers, government officials, policemen, gendarmes, prison guards, osadniks [Polish veterans settled in the Kresy; a sort of colonist] and intelligence agents - by nationality, over 97% of which are Poles.
"Of them, there are:
- Generals, colonels, and lieutenant-colonels - 295
- Majors and captains - 2,080
- Lieutenants, sub-lieutenants and warrant-officers - 6,049
- Officers and junior commanders of the police, border patrol, and gendarmerie - 1,030
- rank-and-file policemen [constables in UK, officers in US], Gendarmes, prison guards, and intelligence officers - 5,138
- Officials, landowners, clergymen and osadniks - 144
"In the prisons of the western oblasts of Ukraine and Belarus are contained a total of 18,632 prisoners (of which 10,685 are Poles), including:
- Former officers - 1,207
- Former police intelligence officers and gendarmes - 5,141
- Spies and diversionists [saboteurs] - 347
- Former landowners, industrialists, and officials - 465
- Members of various counter-rev. and insurgent organizations and other counter-rev. elements - 5,345
- Deserters - 6,127
"Based on the fact that they are entrenched, incorrigible enemies of the Soviet state, the NKVD of the USSR considers it necessary:
- To suggest the NKVD of the USSR:
1. For the cases of those 17,000 persons in POW camps that are former Polish officers, officials, landowners, policemen, intelligence agents, gendarmes, osadniks, and prison guards,
2. As well as the cases of those detainees held in prisons in the western oblasts of Ukraine and Belarus numbering 11,000 people who are members of various counter-rev. spying and sabotaging organizations, were former landlords, industrialists, former Polish officers, officials, and deserters -
- to try [and convict], in a special manner, these persons with the application of capital punishment - by gunshot. [emphasis my own]
- Trials be performed without summoning the arrested nor with laying charges against the accused, ordinances on the termination of the investigation and final conviction are to be as follows:
a) For persons in POW camps - according to information provided by the Office for Prisoners of War of the NKVD of the USSR,
b) For persons arrested - according to information from the cases submitted by the NKVD of the Ukrainian SSR and Belorussian SSR.
- Trying and deliberating to be entrusted to a troika [counsel of three] consisting of Comrades [name scratched out], Merkulov, [added in in pen] Kobulov, and Bashtakov (Head of the 1st Special Department of the NKVD of the USSR).
"Peoples Commission of Internal Affairs
Union of SSR
[Beria's Signature]
L. Beria"
Here are high resolution scans of the document:
[url]http://upload.wikimedia.org/wikipedia/commons/c/c1/Katyn_-_decision_of_massacre_p1.jpg[/URL]
[url]http://upload.wikimedia.org/wikipedia/commons/7/72/Katyn_-_decision_of_massacre_p2.jpg[/URL]
[url]http://upload.wikimedia.org/wikipedia/commons/8/87/Katyn_-_decision_of_massacre_p3.jpg[/URL]
[url]http://upload.wikimedia.org/wikipedia/commons/2/22/Katyn_-_decision_of_massacre_p4.jpg[/URL]
Feel free to correct any errors