In addition to the four gunmen who were quickly detained, dozens of other alleged plotters have been rounded up amid an ongoing investigation. The Kremlin says it is probing Ukrainian and potential US links to the deadly attack.

Moscow previously announced "substantial evidence" of the attackers' financial links to Ukraine involving cryptocurrency transfers. President Putin has acknowledged that 'Islamic extremists' were behind it, yet said some other entity or even state was likely directing them.

In fresh statements translated to English in Russian state media, Putin said of the terrorists, "They're doing anything for money, not guided by any religious or political considerations, only financial ones." He added that "any information can be easily bought and sold" in such a market. He vowed "We will definitely get them" -- in apparent reference to those who ultimately masterminded the plot.

Washington has vehemently rejected any accusations against either Ukraine or the US or Western governments. Instead, it says the Biden White House tried to warn Russian authorities.

However, on Tuesday Russia's spy chief, FSB head Sergei Naryshkin stated that US warnings were too broad and ambiguous. There wasn't enough info to prevent any attack, he said.

"The Federal Security Service [FSB] received some information from the U.S. intelligence services that such a thing was unfortunately possible," Naryshkin told a press briefing. "But as our Russian colleagues said, the information was too general and did not allow us to fully identify those who committed this terrible crime," he was cited by Interfax as saying.