Two Ukrainians have been identified as the suspected perpetrators behind two acts of sabotage targeting a railway line between Warsaw and Lublin on Monday, Polish Prime Minister Donald Tusk told the parliament on Tuesday. According to him, the suspects sought to provoke a train crash.

The prime minsiter accused the suspects of working “with the Russian intelligence for a long time.” According to Tusk, both alleged perpetrators fled to Belarus after the incidents.

A military-grade C4 explosive charge was used in a least one of the incidents, the prime minister said, adding that a 300-meter-long cable was used to detonate it. The National Prosecutor’s Office also confirmed that a cable “that was most likely used to set off the explosive” was discovered.

Another incident involved a steel clamp on a track to cause a derailment, Tusk said. The alleged perpetrators also left a smartphone with a power bank at the scene to record a potential incident, he added.

The prime minister called the two incidents “the most serious” security situation over the past years. “A certain line has been crossed,” he said.

The C4-like explosives were originally developed by the British during World War II and reintroduced as Composition C family by the US military. The C4 variant was developed in the US in 1950s. Russia does not produce C4 explosives and relies on its own types of plastic explosives known as PVV family that were developed back in the USSR.

In September, Moscow warned that Kiev could be planning false-flag operations in Romania or Poland to frame Russia for them. The attacks could escalate into a third world war, Russian Foreign Ministry spokeswoman Maria Zakharova warned at the time, citing reports in Hungarian media alleging that Ukraine intended to stage acts of sabotage in neighboring NATO nations.