President Vladimir Putin said on Tuesday (April 2) that Russia will find out who was behind last month’s attack on a concert hall near Moscow that killed at least 144 people.

“We will definitely get to those who ultimately ordered it,” Putin told a meeting of Interior Ministry officials.

“We paid a very high price, and the entire analysis of the situation must be extremely objective and professional.”

Putin also said that the tragedy at the Crocus City Hall must not be allowed to lead to xenophobia and Islamophobia in Russia.

Islamic State claimed responsibility for the attack and U.S. officials say they have intelligence showing it was carried out by the network’s Afghan branch, Islamic State Khorasan (ISIS-K).

Russia says it has evidence the attackers had links to Ukraine, which Kyiv has denied and the United States has rejected as nonsense.

Russia lowered flags to half-mast for a day of mourning and charged four men it accused of gunning down scores of people at a concert outside Moscow on Friday night in the deadliest attack inside Russia for two decades.

President Vladimir Putin declared Sunday a national day of mourning after pledging to punish all those behind the attack, in which 137 people were killed, including three children, and 182 were injured.

Over 100 people remained in hospital, some of them in a serious condition.

Earlier on Sunday, people laid flowers at Crocus City Hall, the 6,200-seat concert hall outside Moscow where four armed men burst in just before Soviet-era rock group Picnic was to perform its hit “Afraid of Nothing”. The men fired their automatic weapons in short bursts at civilians who fell screaming.

(Reuters)