Two men have been charged in connection with Friday's attack on a concert venue just outside Moscow. Authorities now say at least 137 people died after four gunmen fired at concert goers before setting the building on fire. Well, today was declared a National Day of Mourning for the victims. The Islamic State Group has released a graphic footage which appears to back up its claim of responsibility. But Vladimir Putin has suggested Ukraine was involved. Here's our Russia editor, Steve Rosenberg.

In Moscow, wherever you looked, wherever you turned, you could see this was a country in mourning and in shock. Across Russia, flags flew at half mast for the victims of the concert hall massacre. And at the scene of Friday's attack, Crocus City Hall, the queues grew longer and longer. The National Day of Mourning experienced most acutely here. There was an outpouring of sympathy, a mountain of tributes to the dead. As well as leaving flowers and candles, people have also been bringing soft toys and sweets because amongst the dead, there were children. They've also been leaving messages. One is addressed to the attackers, and it reads, You are scum. We will never forgive you. Among the crowd, there was a mixture of grief and anger.

It was a big shot because I live nearby and I saw heat from my windows. It's horrific and a big tragedy.

Such a terrible loss says Natalia, I'll never forgive or forget. Bring back the death penalty, Evgeny says, for terrorists, for sure. This is how the attack had begun, with gunmen in the foyer and desperate attempts to take cover. The attackers moved on to the auditorium. By the end of this, more than 130 people were dead. Four suspects have been arrested. Tonight, the suspected gunman appeared in court. Russia claims they'd been caught, fleeing to Ukraine, and had contacts there. Kyiv fiercely denies any link to the attack. The Islamic State Group says it was behind the shooting. And on this day of morning, silent prayers from a somber-looking President Putin. The Kremlin says he lit a candle for the victims in Church at his country estate. And Russia is praying that there will be no more attacks. If there are, that will test people's faith in the President to keep them safe. Steve Rosenberg, BBC News, Moscow.

Well, tonight, France has raised its terror threat level in response to the Islamic State Group's claims of responsibility for the attack in Russia. Joining me now is our Security Correspondent, Frank Gardner. Frank, what can we say about why Islamic State might have targeted Russia?

Okay, well, there's three reasons, really, three possible causes. One is Syria, where the Russian Air Force has been inflicting pretty heavy casualties throughout Syria's civil war on Islamic State fighters. A recent communicate by Islamic State's leadership said, We will never forgive and forget. In other words, we will settle scores. That's one possibility. Another is Afghanistan, where ISIS is what's called Khourousan Branch, are fighting the Taliban. Now, that's going to sound strange, but they are mortal enemies. And ISIS considers that yet the Taliban are allies of Russia. In 2022, they attacked the Russian embassy in Kabul. So that's another possibility. And the third is the general suppression by Russia's FSB, that's the domestic security agency, of suspected jihadists and other Muslim individuals inside Russia.

Based on this news tonight from France, Frank, what's your assessment of the Islamic State threat in Europe?

Well, it never went away. Mi5, for for example, here in the UK, which is the security service, their highest priority, the bulk of their workload, is still Isis and Al Qaeda-inspired terrorism, trying to head off those threats. Now, currently, Britain is at the middle of five levels of terror threat. It's at something called substantial, which means that it's not particularly reassuring. A terrorist threat is thought likely, but it's only at the middle one. France has gone to the highest of three. Now, that would only happen in this country if there was an actual evidence of a terrorist plot, which there isn't at the moment.

Okay, Frank, for now, thank you very much. Frank Garner, our security correspondence.