"Besides the pre-trial investigation launched in the Republic of Poland, we also have information from US colleagues that the Federal Bureau of Investigation has also started its one investigation. We are also in active cooperation with colleagues from Ireland and Greece. It is happening everyday, 24/7," Kiskis told journalists on Friday.

In his words, all crew members of the Ryanair plane and "around 90 percent of all passengers" jave been questioned.

Gintas Ivanauskas, a deputy prosecutor general, says the Ryanair plane is still at Vilnius Airport since not all action planned as part of the ongoing investigation has been made. Once it is done, the plane will be able to leave. Additional plane inspection is planned over the next several days.

According to Ivanauskas, a joint investigation team is now being formed with Polish prosecutors as the country is also carrying out a pre-trial investigation into the incident since the plane is registered in Poland.

Kiskis says the FBI launched its own investigation probably in response to information that American citizens were among passengers on the plane.

"Naturally, since they probably have information that their citizens were among the passengers, the country makes its own decision on whether to launch an investigation or not," he said.

The Criminal Police Bureau chief said Vilnius Airport received an alleged letter on a bomb placed on the plane but that information was later found not to be true.

"They received the letter, but I cannot say when. We are checking and verifying the information," Kiskis said.

In his words, a response from Belarus was also received via the Interpol channel on Thursday. "It is very short and says that the plane landed in Minsk and that five passengers were taken off the plane and stayed in Minsk, including Raman Pratasevich who is under pre-trial investigation and was detained as part of a pre-trial investigation being carried out by Belarusian law enforcement," Kiskis said.

Both Ivanauskas and Kiskis confirmed that a legal assistance request will be sent to Belarus over the upcoming days.

Last Sunday, a Ryanair flight en route from Athens to Vilnius changed course just before crossing into Lithuanian airspace and was diverted to Minsk where it landed. Belarusian opposition activist, blogger and founder of the Nexta information channel Raman Pratasevich was detained after the plane landed in Minsk, alongside his girlfriend Sofia Sapega, a Russian citizen and student of the Vilnius-based European Humanities University.

In response to this incident, EU leaders later banned Belarusian airlines from flying into the block and urged EU airlines to avoid the Belarusian airspace. The EU is also considering introducing sectoral sanctions to step up pressure on Belarus.