Anti-terrorist officers were battling against horrific conditions today as they tried to recover bodies and gather crucial forensic evidence from the scene of the Russell Square tube blast.
Fumes, vermin, the risk of asbestos, stifling heat and the threat of the tunnel collapsing have all seriously hampered the recovery operation and made life near-impossible for emergency workers.
One senior security source today described the scene inside the tunnel as "very bad", while a senior British Transport Police officer said it was "carnage".
The bodies of around 20 people remain inside the carriage of the Piccadilly line train, which is about 500 yards from the station inside the enclosed tube tunnel.
A senior security source said today that the first priority had to be to get them out, before detailed forensic examination of the carriages could begin in earnest.
He said the extensive damage to the tunnel - which is at risk of collapse - and the train itself, meant it might have to be examined in situ or even dismantled before it could be looked at properly by forensic experts.
One option under consideration is to detach four or five carriages and tow them away to enable emergency workers to better access the front carriage, which was where the bomb detonated.
It is a massive logistical challenge for police and London Underground's engineers and it is likely to be several days, if not much longer, before the affected section of the tube line is reopened.
"The priority at this stage is to recover the bodies from the scene - and they are very difficult scenes, particularly at Russell Square," the security source said.
"The carriage is some 500 yards into the tunnel. The conditions are very, very difficult. It is a very bad scene down there."
The security services and anti-terrorist chiefs believe that meticulous forensic work could prove to be the key to tracking down the bombers.
Officers will take swabs to ascertain the type of explosive used, before undertaking a detailed forensic search for the bomb components.
"We have to work on the assumption that those who did this are still out there and that they could do it again," the security source said.
"Potentially the forensic work there could save lives in the future."
Deputy Chief Constable Andy Trotter of British Transport Police today said it would take emergency workers "some time" to recover the bodies from the Russell Square blast site.
"There are no living people in there and the challenge is now to remove the dead," he said.
Chief Superintendent Willie McCafferty, British Transport Police commander at King's Cross station, added: "A body recovery operation is going on at the moment. As far as we are aware 21 people died in the incident between King's Cross and Russell Square. That operation will take as long as it takes."