A citizen from Uzbekistan has been detained over the killing of Russian nuclear forces general Igor Kirillov, a Russian intelligence agency has said.
Russia's Federal Security Service (FSB) says the suspect has told them he was recruited by Ukrainian intelligence services and was paid $100,000 (£79,000).
The FSB didn't name the man but said he was born in 1995.
General Kirillov, 54, was killed on Tuesday by a bomb hidden in an electric scooter outside an apartment block in Moscow, a day after Ukraine opened a criminal case against him.
A Ukrainian official said Ukraine's Security Service, or SBU, carried out the attack, in which the senior general's assistant also died.
Dashcam footage at the scene captured the moment the bomb detonated on the street as the two men walked out of the building about 4 miles (7km) southeast of the Kremlin.
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A statement issued on Wednesday by the Investigative Committee of Russia said that under interrogation, the suspect admitted Ukraine had supplied him with a homemade bomb after he arrived in the Russian capital.
He placed the device on an electric scooter, which he parked at the entrance to General Kirillov's home, monitoring the scene via a surveillance camera installed in a hire car, Russian authorities said.
Footage from the camera was watched by "the organisers of the terrorist attack in the city of Dnepr", and the device was detonated after it was confirmed the soldiers had left the building's entrance, the committee added.
Russian foreign ministry spokeswoman Maria Zakharova said everyone involved in the killing would be found and punished.
If convicted, the suspect could be jailed for life.
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As well as the money, the man was offered the chance to move to Europe in return for his part in the assassination, the Russian committee said.
Photographs posted on Russian Telegram channels showed two bodies lying in the blood-stained snow next to the wrecked entrance to a building after the blast.
Russian news agency TASS said the bomb contained the equivalent of 300g of high explosive.
Several cars and the first four floors of the apartment building were damaged, the news agency added.
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General Kirillov was in charge of Russia's radioactive, chemical, and biological defence soldiers, known as RKhBZ. These troops are special forces who operate when there is a threat from radioactive, chemical, and biological contamination.
He was under sanctions from several countries, including the UK and Canada, for his actions in Russia's full-scale invasion of Ukraine.
On Monday, a Ukrainian prosecutor opened a case against General Kirillov in absentia, accusing him of using banned chemical weapons in Ukraine, the Security Service of Ukraine said.
It was claimed that General Kirillov was "using dangerous chemicals mainly in the hottest areas of combat, where they [Russia] are trying to hide the use of chemical agents under dense artillery fire".
Russia denies those accusations.
The e-scooter, which Sky News' Data and Forensics team has geolocated to the scene outside a residential building in Moscow, is from KugooKirin.
The brand was founded in China but has multiple overseas warehouses across Europe, including in Russia.
Russia is also one of its main exporting countries, according to the company's website.