"To shake up the situation, create panic in society," Bortnikov told reporters, when asked about the West’s and Kiev’s interest in the attack.
Bortnikov added that the FSB, along with partner intelligence services of friendly countries, are identifying all participants in the deadly terrorist attack at Crocus City Hall near Moscow.
"Now, a lot is being done to identify all participants in this massacre, both here in the country and naturally abroad, in the process of further operational and search activities, which is what is being done. And all our operational units are actively engaged in this work, including with partner intelligence services of friendly states," Bortnikov said.
Perpetrators of the attack wanted to escape alive after committing it, the FSB head said, adding that it is a fact that they were planning to escape to Ukraine, but the Russian special services did everything to prevent this from happening.
He went on to elaborate that the Ukrainian special forces are involved in the terrorist attack on the Crocus City Hall concert venue, while the Islamic State* militants prepared it.
"We believe that the attack was prepared by both the radical Islamists themselves, and naturally the Western intelligence services contributed to this, and the Ukrainian intelligence services themselves are directly involved in this," Bortnikov said in an interview with Rossiya 1 reporter Pavel Zarubin, adding that a terrorist threat in Russia "persists."
The head of Ukraine's intelligence service, Kyrylo Budanov, can become a legitimate target of the Russian armed forces, Bortnikov noted.
"Those who commit crimes against Russia and Russian citizens are legitimate targets," Bortnikov told reporters.
The information transmitted by the United States on the preparation of a terrorist attack was of a general nature, and the Russian special services responded to it.
"This information about the preparation of terrorist attacks in places of mass gathering of citizens was of a general nature. We responded to this information and, of course, took appropriate measures to prevent such manifestations. But, unfortunately, our actions that we carried out in relation to a specific group and specific individuals, this information was not confirmed at that time", Bortnikov told reporters.
The mastermind behind the terrorist attack is not identified yet, but response measures will be taken following the tragedy, the official added.
When asked if the US, the UK and Ukraine are behind the attack, he said that the Russian special services believe so, adding that Russia is aware that Ukraine trained militants in the Middle East.
"We think so. At least we’re talking about the overall background we have. This, so to say, is general information, but there are already certain developments [in the matter]," he noted.
Russia needs to recognize the Security Service of Ukraine (SBU) as a terrorist organization, Bortnikov also said.
"We have to do this, of course," Bortnikov commented.
*Islamic State (also known as ISIL/IS/ISIS) is a terrorist group outlawed in Russia and many other countries.