Updated

Polish prosecutors who are investigating the 2010 plane crash that killed President Lech Kaczynski and 95 others have sent samples from the plane for laboratory tests in Britain to help them determine whether an explosion caused the disaster.

Ewa Bialik, spokeswoman for the National Prosecutor's Office, said late Thursday that the samples have arrived at Britain's Defence Science and Technology Laboratory. They are to be checked for traces of explosives, a process expected to take a few months.

More samples were also to be sent to laboratories in Italy and in Spain. The samples were taken by Polish investigators who went to Russia after the crash. Russia is refusing to return the wreckage, saying it is needed for an ongoing investigation there.

The prosecutors are investigating whether anyone should face charges over the crash near a rudimentary military airport in Smolensk, Russia. They plan to question Poland's prime minister of the time, Donald Tusk, who is now the president of the European Council.

Poland's ruling party is led by the late president's twin brother, Jaroslaw Kaczynski, who rejects findings by separate teams of Polish and Russian aviation experts who have concluded that the crash resulted from errors by the crew while attempting to land in dense fog.

The prosecutors have brought negligence charges against two officers of the now-dissolved regiment that was responsible for flying top officials. They have also brought charges against two Russian flight controllers from Smolensk for allegedly giving the plane's crew poor guidance, but Poland has no legal power to bring them to trial.