Russia's FSB security service said on April 1 that four people arrested over a foiled "terror" plot had provided money and arms for the deadly attack on a Moscow concert hall last month.
More than 140 people were killed when gunmen stormed the Crocus City Hall venue on March 22 before setting the building on fire in the most deadly attack in Russia for two decades.
The FSB said in a statement Monday that it had arrested four people a day earlier in the southern Dagestan region who "were directly involved in the financing and supply of terrorist means to the perpetrators of the terrorist act carried out on 22 March in the Crocus City Hall in Moscow."
Moscow terror attack: Why did ISIS target Russia? | In Focus podcast
Russia's national anti-terrorism committee said on Sunday that it had apprehended three people who were "planning to commit a series of terrorist crimes."
The FSB said Monday that four foreign citizens had been arrested in the operation in the regional capital Makhachkala and the nearby town of Kaspiysk.
The Interfax news agency cited an FSB video showing one of the detained men saying: "I took weapons to them, these guys who attacked Crocus City Hall. I took them weapons from Makhachkala."
Russian authorities had previously announced the arrests of 12 people they say are connected to the attack -- including the four suspected gunmen, who have been identified as Tajik citizens.
Ten suspects charged
The Investigative Committee, which probes major crimes, said separately Monday that it was formally charging another suspect with carrying out a "terrorist attack".
"At the moment, the investigation has charged ten defendants," it said in a statement.
Moscow courts confirmed the tenth suspect, Yakubdjoni Yusufzody, has been detained until May 22, accused of having "transferred money to an accomplice" a few days before the attack to "provide accommodation for the terrorists".
Citing information provided to the court, Russian agencies reported that Yusufzody is also from Tajikistan.
The Islamic State (IS) has claimed responsibility for the massacre, the most deadly it says it has ever carried out on European soil, though President Vladimir Putin has talked up a Ukrainian and Western connection.
Kyiv and the West have repeatedly denied any involvement and accused Moscow of "exploiting" the tragedy.
Dagestan is a Muslim-majority region in Russia's southern Caucasus region.
The FSB has come under scrutiny over its failure to thwart the attack despite private and public warnings by the US intelligence community that "extremists" were planning an "imminent" attack on "large gatherings" in Moscow.
The agency regularly announces it has foiled alleged "terrorist cells", but in recent months has mainly announced the arrests of what it calls pro-Ukrainian saboteurs planning attacks on Russian military sites and infrastructure.
