A buried point of clarification: This is the last consulate to close, but the Warsaw embassy remains open.

Poland’s foreign minister, Radosław Sikorski, has described last weekend’s sabotage attack on Poland’s rail system as “an act of state terrorism” ordered by Russia, as he announced that Poland was closing the last remaining Russian consulate in the country.

“The clear intention was to cause human casualties,” he said of the weekend bomb attack.

The Polish security services said they were in the process of arresting several people linked to the incident.

“They are on the trail of the principals; they are on the trail of the perpetrators … the first arrests are now taking place,” the spokesperson Jacek Dobrzyński said on Wednesday morning. He said later that four people had been arrested.