Polish Prosecutor General's office said on its website it was looking into four preliminary versions of the recent air crash that killed President Lech Kaczynski.
Polish president Lech Kaczynski, his wife, and an official Polish delegation of senior officials died when the plane taking them to a ceremony to pay tribute to some 20,000 Polish officers murdered by Soviet secret police crashed near west Russia's Smolensk on April 10.
The Prosecutor General's office said in a communique on Thursday that "a technical failure," "a crew mistake," "poor flight management and security" and "third party's actions," such as a terrorist attack or pressure on the crew, were seen as possible causes of the crash, in which 96 people were killed.
Previous media reports ruled out a malfunction in one of the plane's systems.
One of the most popular versions is that Kaczynski ordered the pilot to land in thick fog, although air traffic controllers gave no permission to land and recommended the crew to divert to another airport.
Poland's chief prosecutor Andrzej Seremet earlier said that there was no proof that the pilot of the ill-fated plane was pressurized into performing a landing in poor weather conditions.
The communique said there was no evidence confirming pressure on the crew at the moment, but "it was too early to rule out this version."
Russian investigators, experts and Polish specialists are jointly investigating the causes of the deadly crash and Polish military prosecutors have opened their own probe.
Russian Prime Minister Vladimir Putin and his Polish counterpart Donald Tusk discussed in a phone conversation the investigation into the plane crash and expressed satisfaction about the level of cooperation between the Russian and Polish investigating bodies.
MOSCOW, April 30 (RIA Novosti)
