There were non-crew members in the cockpit of the Polish presidential plane that crashed near Smolensk on April 10, Chief of the Interstate Aviation Committee (IAC) Tatyana Anodina told local media on Wednesday.

"It was proven that some of the persons in the cockpit were not crew members," she said, "The voice of one of them has been identified, while the others are being identified by the Polish side."

Anodina said that the voices were difficult to identify because the door to the pilot's cockpit had been open.

"Unique equipment was used to clear the recording of the noises. As a result, the voices were identified quite distinctly," she said.

According to the findings from Russian investigation, there was no terrorist attack, fire, explosion or engine problem on board of the plane. The pilot had no navigation and weather data before he took off from Poland, but he was repeatedly informed about the weather in the area before landing.

"The air traffic controller twice warned the crew about fog and 400-meter visibility and said there were no conditions for landing," Alexei Morozov, head of the IAC technical commission, told the Interfax news agency.

The causes of the plane crash will be established and named within the tightest deadlines, Anodina said.

"The results will be open and made available to the public at large. Also, they will be handed over to the Russian and Polish teams of inquiry and the prosecutor-general's offices of the two countries," she said, "I am confident that all the circumstances and cause of the catastrophe will be established objectively and openly."

The plane carrying late Polish President Lech Kaczynski crashed during landing in Smolensk in western Russia, killing all 96 persons onboard. The Russian and Polish governments earlier agreed to co-investigate the crash.