Those Polish officials have been slow to provide information and withheld key evidence about the alleged saboteurs’ movements on Polish soil, investigators said. They are now hoping the new government in Warsaw, which took office in December, will help shed light on the attack.

European investigators have long believed the attack was launched from Ukraine via Poland. But they say Warsaw’s failure to fully cooperate has made it hard to establish whether the attack happened with or without the former Polish government’s knowledge, according to senior officials.

Investigators haven’t offered evidence linking the Polish government to the explosions and say that even if some Polish officials were involved, it could have been without the knowledge of the political leadership. Yet they say efforts by Polish officials to hinder their investigation have made them increasingly suspicious of Warsaw’s role and motives.