At least 14 people were killed and dozens more were wounded Monday after an explosive device blew up inside a subway train car in St. Petersburg, according to multiple reports. Authorities in the Russian city are investigating whether terrorism played a role in the deadly blast.

Here’s what we know about the incident:

What happened?

An “unidentified object” detonated inside a train carriage at about 2:20 p.m. local time while the car was traveling underground between two stations, the St. Petersburg Metro said in a statement, according to the AFP. Officials confirmed that at least 14 people were killed and more than 40 others were wounded, the Associated Press reported. Authorities said 49 people had been hospitalized. The identities and ages of the victims have not yet been released.

While there was no immediate claim of responsibility for the attack, authorities in Kyrgyzstan have since said that a Kyrgyz-born Russian national named Akbarzhon Dzhalilov has been identified as a suspect, according to the AP. Russian investigators said forensic experts found Dzhalilov’s DNA on a bag containing a bomb that was found and deactivated hours later at another subway station in St. Petersburg on Monday.

The investigators have also identified a Kyrgyz-born man as a suicide bomber who was behind the attack. The 22-year-old’s body fragments were found in the train. It is unclear whether the man in question is Dzhalilov and whether or not the figure of 14 deaths includes him, the AP reports.

Was it a terrorist attack?

The explosion is under investigation. It’s unclear whether the device was left in the car purposefully when it exploded between the Sennaya Ploschad and Tekhnologichesky Institut stations. Authorities later found and deactivated a bomb at another St. Petersburg subway station, according to the AP, which cited a Russian anti-terrorism committee. The city is considered Russia’s most popular tourist destination, and its metro system is the 19th busiest in the world, the BBC and AP reported. More than 2 million people travel through it daily.

Russian President Vladimir Putin, who was in the city at the time of the blast, said investigators are looking into all causes of the explosion, including terrorism, according to the BBC. It’s unclear if authorities have any leads or suspects yet. “Law enforcement agencies and intelligence services are doing their best to establish the cause and give a full picture of what happened,” Putin said, according to the AP.

U.S. President Donald Trump said the bombing was “absolutely a terrible thing” in comments on Monday. “It’s happening all over the world,” he said. The Kremlin said Trump called Putin to offer his condolences to the families of the victims, the AP reports. A statement from the Russian government reportedly said that the two leaders shared the view that “terrorism is an evil that must be fought jointly.”

What happens next?

All subway stations in St. Petersburg have been closed, and authorities have tightened security in Moscow’s major transport hubs, the AP reported.

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