Russian intelligence services were likely behind a series of packages that exploded in warehouses of courier companies in various parts of Europe, Polish prosecutors said today, blaming Moscow in a case that officials say could have caused an airline tragedy.

Western officials have said in the past that Russia has tried to destabilize Ukraine’s allies with acts of sabotage. Explosions at warehouses of courier companies in Britain, Germany and Poland in July could signal a major escalation by causing an aircraft explosion.

Security officials have said the packages were part of a plot ultimately aimed at detonating commercial flights to the US and Canada.

“The evidence gathered in the case indicates a high probability that the planned acts of sabotage … were inspired by the Russian special services,” Polish prosecutor’s spokesman Przemysław Nowak said in an emailed statement.

The Russian embassy in Warsaw did not respond to a request for comment. Russia has previously denied any involvement.

Nowak said Polish authorities are looking for two Russians and have detained four Ukrainian citizens as part of the investigation.

Polish prosecutors said in October that they had detained four people and were looking for two others in connection with the packages, but did not disclose the nationalities of those accused of involvement.

In October, prosecutors said they were investigating the role of “foreign intelligence agencies” without specifically naming Russia.

According to Novak, intensive actions are being taken to identify the other persons involved in the case. “These actions are carried out in cooperation with the law enforcement agencies of other countries of the European Union.”

He said that the packages with the explosives had been sent from the capital of Lithuania, Vilnius.