What's happening in St Petersburg?
An investigation has started after an explosion on a train in the Russian city of St Petersburg.
Fourteen people died and another 49 were injured on Monday afternoon.
St Petersburg is the second largest city in Russia.
The explosion happened as the train travelled between two underground stations.
The driver continued to the next stop, which allowed passengers to be helped more easily by emergency services. His actions have been praised as helping to save further lives.
Russia's emergency services quickly arrived on the scene and took people to hospital.
The St Petersburg metro system was shut down and three days of mourning were declared.
Another device was found at a different metro station, but was safely defused.
The Russian government has said it is introducing extra security measures.
Who is responsible?
Details are still coming out about who was behind the bomb.
The Russian Prime Minister, Dmitry Medvedev, has described the explosion as a "terrorist act", but officials say they are investigating all other possibilities too.
So far, no group has said it carried out the attack.
Russian authorities have said they have identified a suspect, who is thought to have also died in the explosion.
How has the world responded?
The Russian President Vladimir Putin was in St Petersburg - his home city - when the blast happened.
He visited the scene on Monday evening and laid flowers at a makeshift shrine.
People lit candles and placed flowers at the entrance to the metro station following the incident.
A service was held at a chapel in Russia's capital city Moscow, with prayers for those affected by the blast.
Many world leaders have condemned the attack.
UK Prime Minister Theresa May said she had written to President Putin to offer her condolences.
"I'm very clear, as the people of the UK were after the attack in London, that we will prevail. The terrorists will not win."
The White House said US President Donald Trump had spoken to Mr Putin by phone and offered "full support" in bringing those responsible to justice.
What should I do if I'm worried?
It's important to remember that things like this are still extremely rare - that is why they're in the news.
If you're feeling worried then it's best to talk to someone you trust, like a parent or teacher.
There's more advice here on what to do if you're upset by the news.