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 initially said that the bomb used up to 300g of explosive material, before later revising that figure up to 1kg, citing law enforcement data.

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.

Additional reporting by Jemimah Herd.