MOSCOW (AP) — Russian President Vladimir Putin vowed Tuesday to track down the masterminds of the Moscow concert hall attack that left 144 people dead in the worst assault on Russian soil in two decades, and urged the country’s law enforcement agencies to tighten security at mass gatherings.

Putin has repeatedly sought to link the March 22 attack to Ukraine and the West despite the Islamic State group’s claim of responsibility and Kyiv’s vehement denial.

Speaking at a meeting with top officials of the Interior Ministry that oversees the nation’s police force, Putin said it is important to determine “not only the perpetrators of this outrage, but all links in the chain and its beneficiaries.”

He added, in an apparent threat of retaliation: “Those who use this weapon against Russia should realize it’s a double-edged weapon.”

Putin said that the masterminds of the concert hall raid sought to “sow discord and panic, strife and hatred in our multiethnic country in order to break up Russia from within,” adding that “we mustn’t allow them to do that.”

“It’s inadmissible to use the tragic event to provoke ethnic tensions, xenophobia and Islamophobia,” he said.

Russian security agencies have detained four suspected gunmen, all of them citizens of Tajikistan, and seven other suspects.

The attack has fueled ant-migrant sentiments and drawn calls from Russian hawks to limit migration despite the fact that the Russian economy strongly depends on migrant workers, most of them citizens of ex-Soviet nations in Central Asia, including Tajikistan.

Russian media reported that authorities have strengthened controls over migrants in the wake of the attack.

Putin urged the Interior Ministry to tighten controls to stop illegal migration and close loopholes in existing procedures that allow people with a criminal past get work permits and even Russian citizenship.

The lapse in security has led many to wonder how gunmen could easily kill so many people at a public event. Kremlin critics have argued that the security blunder was rooted in the Russian vast security apparatus focusing heavily on stifling the political opposition, independent media and civil society groups in the harshest crackdown since Soviet times instead of on terror threats.

Putin said that the authorities are investigating the performance of law enforcement structures and other agencies in the concert hall attack. He urged law enforcement agencies to strengthen security at public gatherings.

“We have paid a very high price, and the analysis of the situation must be objective and professional,” he said. “We need to take ensuring security and order at mass gatherings, sports facilities, transport, trade and recreational centers, schools, hospitals, universities, theaters and so on. All those facilities must be under constant control.”

Putin, who has repeatedly accused the U.S. and its allies of trying to destroy Russia, again charged that Moscow’s geopolitical adversaries were aiming to “ruin what is left of historic Russia, to break up its historic core” in order to win control over the country’s vast resources.

“Some of them are trying to preserve their hegemony in today’s rapidly changing world at the expense of Russia,” he said. “Some apparently saw our country as a weak link. They are mistaken.”

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