It was a brazen pre-dawn attack in the heart of Moscow.
As a top Russian military general and his assistant exited an apartment building in Russia's capital, danger lurked nearby.
A bomb had been placed outside the complex on Ryazansky Prospekt, a street near the Kremlin, according to Russia's main federal investigative authority.
It was reportedly concealed inside an electric scooter.
When Lieutenant General Igor Kirillov and his colleague swung open the doors and walked outside on Tuesday night, the bomb was remotely triggered to explode.
Ukrainian sources almost immediately confirmed the country's intelligence service, the SBU, was responsible for the killing. The government has not yet commented.
If the assassination was an act orchestrated by Kyiv, it is another reminder of its ability to strike back despite President Vladimir Putin's invasion of Ukraine in February 2022.
While Kyiv rarely officially claims responsibility for attacks on Russian soil, it is believed to have carried out a string of targeted assassinations since the start of the war.
In the immediate aftermath, the Kremlin has already promised to respond, warning Ukraine it faces imminent revenge for the killing.
But how significant is the death of a top Russian military general and how could Putin respond?
Who was General Kirillov?
General Kirillov oversaw the Russian military's nuclear and chemical weapons protection forces.
The 54-year-old had been in the Russian armed forces since 1987 and was credited with developing the TOS-2 Tosochka heavy flamethrower system.
The weapon can hit a target at a range of over 10 kilometres, making it a feared threat on the battlefield.
When Putin launched his full-scale invasion of Ukraine in 2022, General Kirillov was sanctioned for his alleged use of chemical weapons on the country.
"The UK will not sit idly by whilst Putin and his mafia state ride roughshod over international law, including the Chemical Weapons Convention," UK Foreign Secretary David Lammy said in October when sanctioning General Kirillov.
Russia has been accused of using ammonia and chloropicrin gas on the battlefield in the Donbas.
Chloropicrin was known as a "vomiting agent" in World War I, causing soldiers to experience nausea and diarrhoea for weeks after exposure.
Just hours before his death, Ukraine's Security Service (SBU) charged General Kirillov in absentia for ordering the massive use of banned chemical weapons against the Ukrainian army.
"Since the beginning of the full-scale war, following Kirillov's orders, the Russian army used different types of banned chemical munitions against Ukraine more than 4,800 times," the SBU said in a statement.
Hours later, General Kirillov was dead.
The assassination was caught on camera
Dash cam footage taken from a nearby car on the night of the attack shows two men dressed in black exiting a nondescript brick building through the front door.
One of them is Lieutenant General Igor Kirillov and the other is his assistant.
Next to them is what appears to be a scooter, leaning against the exterior of the apartment block on the right-hand side.
As the doors swing open, the two figures are shown exiting the building and trudging through the snow to a waiting black car.
But before they can make it, the screen erupts into a sudden flash of white.
There is a glimpse of a fiery inferno exactly where the two men were last seen standing on the footpath before it is replaced by a thick plume of smoke.
Photographs posted on Russian Telegram channels in the aftermath of the explosion showed rubble lying around the shattered entrance and two bodies lying in the snow.
The street is now taped off and access is severely restricted as investigators comb the scene for evidence.
Residents, meanwhile, have been left mystified by the brazen attack.
One local resident had initially assumed the noise of the explosion was cement being unloaded, but claimed it was very loud.
"At the moment, you see a lot of police," another local resident said.
"But usually everything is open. Anybody can go in and out, drive in and out."
What will this mean for Russia?
Ukrainian government sources have told local and foreign media, including the BBC and the New York Times, that their spies were responsible for the assassination.
The Ukrainian government has not yet commented on the general's death.
But an unnamed source told the BBC he was a "legitimate target".
This is not the first time Ukraine has been blamed for an assassination on Russian soil.
Last year, a woman approached a prominent war blogger, Vladlen Tatarsky, at a cafe in St Petersburg.
She presented him with a sculpture of a soldier's head that later exploded, killing him and wounding several others.
Ms Trepova, who was sentenced to 27 years in prison, said she did not know she was taking part in an assassination plot, instead insisting she thought the sculpture contained a microphone so Mr Tatarsky's conversations could be monitored.
She said she had acted on the orders of a Ukrainian contact and was set up, something Ukraine has denied.
Security expert at Aberystwyth University Dr Jenny Mathers said Russia has a long history of targeted assassinations and acts of sabotage.
"In many ways, Ukraine seems to be taking a leaf out of Russia's book," Dr Mathers said.
"Ukraine is demonstrating to the Kremlin that it has the ability, has the reach to go right into Moscow and assassinate a prominent senior military figure.
"It's a way of saying to Moscow, 'Two can play at this game.'"
If Ukraine is indeed responsible, Dr Mathers said it would be a psychological blow to the Kremlin to know foreign agents infiltrated the country and killed a top-level target.
Could Russia be plotting revenge on Ukraine?
Dmitry Medvedev, deputy head of Russia's Security Council, said that Russia would retaliate for General Kirillov's assassination.
"Everything must be done to destroy those who ordered the murder of General Kirillov, this military-political leadership of Ukraine," Mr Medvedev said during the Russian state defence procurement meeting.
But Dr Mathers says the alleged assassination is unlikely to change the course of the war, now stretching into its third year.
"Will they be able to follow up these assassinations with something which is really very effective at reducing Russia's capacity to wage the war?" she said.
"It does raise some big questions about how vulnerable, how fragile Russia is."
The attack has also threatened the Kremlin's ability to project an aura of power and might.
"How good is its security if it can't protect someone so senior in the armed forces? What else is wrong there? How else are the strains beginning to show?" she said.
"So I think it should raise questions in sort of the outside world, but it also should create some soul-searching in the Kremlin now."
