Russia on Wednesday scrambled to unravel the possible motives of the alleged bomber behind a blast in the Saint Petersburg metro that killed 13 people, as the grieving city mourned its dead.
Investigators have named the attacker as 22-year-old Akbarjon Djalilov, believed to be a Russian national born in Central Asian Kyrgyzstan, saying he had also planted a bomb at another station that was successfully defused.
Authorities said they had definitively identified all 13 victims of the attack, and had also confirmed Djalilov's fragmented remains using DNA tests.
Investigators announced they found items such as sellotape and tin foil matching those used on the bomb during a search of Djalilov's residence, and said CCTV footage showed him leaving his home ahead of the attack "with a bag and rucksack".
The head of Russia's Investigative Committee Alexander Bastrykin ordered officials to look into any potential "links" between the alleged attacker and the Islamic State (IS) group.
No one has so far claimed responsibility for the attack that tore through a subway carriage on Monday afternoon.
But jihadists from IS -- which includes foreign fighters from ex-Soviet Central Asia and the Caucasus region -- have repeatedly threatened an attack on Russian soil in revenge for Moscow's military backing of Syrian leader Bashar al-Assad.
In the first sign of a crackdown on suspected Islamists since the attack, authorities said they had detained in Petersburg seven suspected "terrorist" recruiters from Central Asia, working for groups including IS, but stressed there was no proof yet of any links to Djalilov.
The alleged attacker's distraught parents have flown to Saint Petersburg from their home city of Osh in southern Kyrgyzstan and were questioned by investigators.
Authorities in the mainly Muslim nation of Kyrgyzstan say Djalilov and his parents are ethnic Uzbeks with Russian citizenship and that Djalilov has lived in Russia since he was 16.
Neighbours at the block of flats where Djalilov was staying in the north of Saint Petersburg said that he had moved there about a month before the attack.
"He was calm and quiet," Darya Folomkina told AFP.
"He didn't make any noise. He had some friends over sometimes. He used to say hello," she said, adding that she had seen investigators carry "some boxes" out of the apartment after searching it.
- Identifying the victims -
As the authorities probed the circumstances of the attack, they also completed the grisly task of identifying the victims and geared up to return their remains to relatives.
The ages of those killed ranged from around 17 to 71 with nationals of Azerbaijan and Kazakhstan reported among the dead.
Some 55 injured people remained in hospital with three in a critical condition, health officials said.
Dozens of people gathered in a Saint Petersburg cathedral for a memorial service for the dead, as Russia marked a second day of national mourning over the attack.
"She was a remarkable, creative person," said the sister of 50-year-old Irina Medyantseva, a dollmaker who died in the blast.
"What happened is terrible," said the woman, refusing to give her name.
The attack has stunned Russia's second city and posed tough security questions as it gears up to host the opening game and final of the Confederations Cup football tournament in June, ahead of the country holding the World Cup in 2018.
Russia suffered a wave of brutal attacks in the 1990s and 2000s blamed mainly on a rebellion in Chechnya that morphed from a separatist uprising into an Islamist insurgency.
The country's transport network -- including the metro in Moscow -- was hit repeatedly by suicide bombers leaving scores dead.
But there had been no attacks against a major city since blasts in the southern city of Volgograd in December 2013, weeks ahead of the Sochi Winter Olympic Games.
IS, however, has struck at Russia abroad, claiming a bomb attack in October 2015 that blew a passenger jet packed with holidaymakers returning to Saint Petersburg out of the air over Egypt, killing all 224 people onboard.
In the wake of the Saint Petersburg metro bombing, President Vladimir Putin held talks with world leaders including US President Donald Trump to push for greater cooperation in the fight against terrorism.
The Kremlin has called the attack a "challenge to every Russian, including the head of state".
Putin raised the issue at a meeting scheduled before the attack with security bosses from the ex-Soviet region in Moscow.
"We see that the situation unfortunately is not improving," Putin said in televised comments.
"We know that each of our countries...is a potential target for terrorist attacks."