Polish war crimes prosecutors are investigating the killing of thousands of members of the country's elite by Soviet secret police in 1940.
Russia admitted responsibility for the massacre in 1989
The massacre - in Katyn Forest near the Russian city of Smolensk - happened while the Poles were held prisoner.
Families of the victims want the killings to be recognised as genocide.
Prosecution team head Leon Kieres said they would try to identify those involved in ordering and carrying out the killings.
The team intends to interview thousands of victims' relatives and examine Russian files on the massacre.
The Katyn massacre has been a sensitive issue in relations between Russia and Poland for more than a decade since the fall of communism in both countries.
'No war crime'
An inquiry into the massacre by Russian military prosecutors was closed in September.
The prosecutors refused to describe the massacre as a war crime or a crime against humanity.
"The terminated Russian inquiry does not in our opinion warrant abandoning further investigations into the Katyn crime," Mr Kieres said, quoted by Reuters news agency.
The Soviet Union and Nazi Germany invaded Poland in September 1939 after Moscow and Berlin signed a secret pact to divide Eastern Europe.
Millions of Poles were arrested by the Soviet secret police and most sent to labour camps.
But more than 21,000 army officers and intellectuals were executed in Katyn and other parts of the USSR.
The Nazis discovered the bodies of several thousand Polish officers at Katyn in 1943, but Moscow did not admit responsibility for the killings until 1990.