Luxembourg’s Grand Duke and Duchess have sent a letter of condolences to Russia’s President Vladimir Putin following the attack in St Petersburg on Monday.
A total of 14 people have been confirmed dead and around 50 have been injured after a bomb exploded in the metro.
Grand Duke Henri and Grand Duchess Maria Teresa wrote a telegram to Mr Putin on Tuesday.
The letter reads: “It is with great sadness that the Grand Duchess and myself learned of the attack on the metro in Saint Petersburg yesterday afternoon.
“We want to express our deepest sympathy and solidarity, in particular to the families of the victims and to all Russian people.”
Russia’s Investigative Committee said on Tuesday they suspected a suicide bomber was behind the attack.
Investigators said in a statement that “it has been established that the explosive device could have been activated by a man whose fragmented remains were discovered in the third carriage” of the train.
It was not immediately clear whether he counted among the dead.