BBC Verify has been analysing social media videos and images from Moscow to try to build up a clearer picture of the blast that killed Russian Lt Gen Igor Kirillov and his assistant.

Ukrainian sources say they carried out the attack on the senior commander, who has overseen Russia's Radiation, Chemical and Biological Protection troops since 2017. He was sanctioned by the UK in October and accused of being a "significant mouthpiece for Kremlin disinformation".

Lt Gen Kirillov was leaving a modern apartment building in south-east Moscow early on Tuesday when the device hidden in a parked scooter was detonated, Russia's Investigations Committee - the country's equivalent of the FBI - said.

Footage authenticated by BBC Verify shows two people leaving the building before an explosion next to the entrance. Their clothes are the same colour as those on the bodies pictured in the aftermath.

A scooter appears to be propped up near the door. Nothing after the blast is visible, the rest of the footage is obscured by debris from the explosion.

In other footage analysed by BBC Verify, the remains of an electric scooter can be seen lying among debris between two bodies and a demolished doorway.

A close-up image of the scooter, verified by matching it to the debris, shows it is largely intact - apart from its missing handlebars.

Experts at Janes - a private open-source investigations organisation - told BBC Verify that the images suggested that the explosion was caused by an Improvised Explosive Device (IED), rather than a conventional munition.

"From the available pictures of the scooter it actually looks pretty intact, so the effect appears to be more fragmentation than blast," a spokesperson with the organisation told BBC Verify.

An IED is a type of "homemade" bomb which can come in a variety of forms and usually contains easily obtainable components - such as nails, glass or metal fragments - according to the US Department of Homeland Security.

David Heathcote, an intelligence manager with security analysts McKenzie Intelligence, also suggested that the device appeared to be an IED.

"IEDs can be deliberately constructed so as to appear as an everyday item that would not look out of place," he told BBC Verify. "In this instance it appears that the device was concealed within a scooter, most likely using a military grade explosive such as (but not necessarily) Semtex, rather than a home-made explosive."

He added that the attack demonstrated "a decent level of sophistication and may have taken months of preparation" which would have required a "significant amount of surveillance of the intended target prior to the attack".

Russian media has speculated that a device had been taped to the scooter's handlebars.

It has also reported, citing law enforcement sources, that the explosion was likely detonated remotely. Mr Heathcote agreed with the assessment, suggesting that a radio signal from a phone or car fob may have been the trigger.

"The attacker will need to have eyes on the device and will press a switch to initiate the explosion as the target comes into range," he told BBC Verify. "The proximity to the device of the attacker depends on the transmitter [or] receiver being used."

Russian state media has said that the bomb used up to 300g of explosive material. According to a UN tool to estimate damage from blasts, 300g of TNT equivalent explosive can break small windows from a distance of about 17m (55ft) away, or cause damage to brick houses from 1.3m away.

An eyewitness has been showing Russian media images of what he said was debris from the explosion that landed in the flat he owns across the street from the site. BBC Verify is showing these to weapons experts to help establish whether they could have come from the device.

According to the developers, the apartment block where Lt Gen Kirillov was killed is relatively new and was completed in 2019. The flats are relatively high end, with some available for around 20m Roubles (£153,540) online.

In the videos verified by the BBC, a small crowd is seen gathered around the bodies, which are lying in the snow-covered street. Emergency services vehicles are also visible in the footage.

Both videos were filmed on a street looking north toward Ryazansky Prospekt, Moscow, and emerged on social media on Tuesday morning. They show damage to the front of the building, including bricks strewn around and doors hanging open, only a few feet from the bodies.

Another video has captured a bright flash slightly off camera. The force of the explosion knocks snow off nearby parked cars.

At least 10 other apartments on the street have been affected by the explosion, the popular Russian Telegram channel Mash reported.

BBC Verify will continue to examine footage from the scene as it comes in and bring updates.